FIELD [0001] Embodiments of the present disclosure generally relate to a technology of communications, and more specifically, to a method for multiple input multiple output (MEMO) communications and a corresponding terminal device and network device. BACKGROUND [0002] At present, a multi-user multiple input multiple output (MU-MEVIO) technology has been proposed for a long term evolution (LTE) system. The MU-MEVIO technology involves allocation of partially overlapping bandwidths or resources to a plurality of terminal devices, and therefore it is further desired to use a technology of Interleaved Frequency Division Multiple Access (IFDMA) in the MU-MIMO to multiplex uplink signals from the plurality of terminal devices. However, a drawback of using the IFDMA is that it poses limitations on resource scheduling of a base station or an eNodeB in the LTE system. For example, the number of physical resource blocks (PRBs) for transmitting data on a physical uplink shared channel (PUSCH) is limited. SUMMARY [0003] In general, embodiments of the present disclosure provide a method for multiple input multiple output (MIMO) communications and a corresponding terminal device and network device. [0004] In a first aspect, embodiments of the present disclosure provide a method for multiple input multiple output (MIMO) communications. The method comprises: receiving, at a terminal device, scheduling information from a network device, the scheduling information at least indicating physical resource blocks shared by a demodulation reference signal (DMRS) of the terminal device and a further DMRS of at least one further terminal device, and different sub-carriers allocated to the DMRS and the further DMRS in each of the physical resource blocks, the number of physical resource blocks being indivisible by a total number of the terminal device and the at least one further terminal device; generating the DMRS, a length of the DMRS being determined based on the number of physical resource blocks and the number of at least one further terminal device; and transmitting the DMRS to the network device on the sub-carriers allocated to the DMRS in the physical resource blocks. [0005] In a second aspect, embodiments of the present disclosure provide a method for multiple input multiple output (MIMO) communications. The method comprises: generating scheduling information, the scheduling information at least indicating physical resource blocks shared by a plurality of demodulation reference signals (DMRSs) of a plurality of terminal devices, and different sub-carriers allocated to the plurality of DMRSs in each of physical resource blocks, the number of the physical resource blocks being indivisible by a total number of the plurality of terminal devices; transmitting the scheduling information to the plurality of terminal devices; receiving the plurality of DMRSs from the plurality of terminal devices on the sub-carriers respectively allocated to the plurality of DMRSs in the physical resource blocks, lengths of the DMRSs being determined based on the number of physical resource blocks and the number of the plurality of terminal devices. [0006] In a third aspect, embodiments of the present disclosure provide a terminal device. The terminal device comprises: a transceiver configured to receive scheduling information from a network device, the scheduling information at least indicating physical resource blocks shared by a demodulation reference signal (DMRS) of the terminal device and a further DMRS of at least one further terminal device, and different sub-carriers allocated to the DMRS and the further DMRS in each of the physical resource blocks, the number of physical resource blocks being indivisible by a total number of the terminal device and at least one further terminal device; a controller configured to generate the DMRS, a length of the DMRS being determined by the number of physical resource blocks and the number of at least one further terminal device; and the transceiver being further configured to transmit the DMRS to the network device on the sub-carriers allocated to the DMRS in the physical resource blocks. [0007] In a fourth aspect, embodiments of the present disclosure provide a network device. The network device comprises: a controller configured to generate scheduling information, the scheduling information at least indicating physical resource blocks shared by a plurality of demodulation reference signals (DMRSs) of a plurality of terminal devices, and different sub-carriers allocated to the plurality of DMRSs in each of the physical resource blocks, the number of the physical resource blocks being indivisible by a total number of the plurality of terminal devices; a transceiver configured to: transmit the scheduling information to the plurality of terminal devices, and receive the plurality of DMRSs from the plurality of terminal devices on the sub-carriers respectively allocated to the plurality of DMRSs in the physical resource blocks, lengths of the DMRSs being determined based on the number of physical resource blocks and the number of the plurality of terminal devices. [0008] Through the following description, it is to be understood that according to the embodiments of the present disclosure, by designing DMRSs based on lengths required for different DMRSs to be transmitted by the terminal devices to the network device in uplink, IFDMA multiplexing of data (for example, including the DMRSs) transmitted from the plurality of terminal devices on a PUSCH channel is implemented for adaptation to the MU-MIMO technology. Meanwhile, it is allowed to transmit data using any number of physical resource blocks on the PUSCH channel so as to eliminate such a limitation on the resource scheduling when the IFDMA is adopted in a MU-MIMO system as allocating to the terminal device only the physical resource blocks having a number which is an integral multiple of the number of terminal devices, that is, which is divisible by the number of the terminal devices. Therefore, band widths or the number of physical resource blocks used by the terminal devices on the PUSCH channel may be more flexible in the case where the IFDMA is employed. [0009] It is to be appreciated that contents described in this Summary are not intended to identify key or essential features of the embodiments of the present disclosure, or limit the scope of the present disclosure. Other features of the present disclosure will become easily comprehensible through the following description. BRIEF DESCRIPTION OF THE DRAWINGS [0010] Through the following detailed description and with reference to the accompanying drawings, the above and other features, advantages and aspects of various embodiments of the present disclosure will become more apparent. In the drawings, same or similar reference signs represent the same or similar elements, wherein: [0011] Fig. 1 illustrates an example communication network in which embodiments of the present disclosure can be implemented; [0012] Fig. 2 illustrates a flow chart of an example method according to some embodiments of the present disclosure; [0013] Fig. 3 illustrates a flow chart of an example method according to some other embodiments of the present disclosure; [0014] Fig. 4 illustrates a block diagram of an apparatus according to some embodiments of the present disclosure; [0015] Fig. 5 illustrates a block diagram of an apparatus according to some other embodiments of the present disclosure; [0016] Fig. 6 illustrates a block diagram of a device according to some embodiments of the present disclosure. [0017] In all drawings, same or similar reference numbers represent the same or similar elements. DETAILED DESCRIPTION [0018] Embodiments of the present disclosure will be described in more details with reference to the drawings. Although the drawings show some embodiments of the present disclosure, it is to be understood that the present disclosure may be implemented in various manners and should not be construed as being limited to the embodiments explained herein. On the contrary, the embodiments are provided for a more thorough and complete understanding of the present disclosure. It is to be understood that the drawings and embodiments of the present disclosure are only for the purpose of illustration, without suggesting any limitations on the protection scope of the present disclosure. [0019] As used herein, the term "network device" refers to a base station or other entities or nodes having a particular function in a communication network. The term "base station" (BS) may represent a node B (NodeB or NB), an evolution node B (eNode B or eNB), a remote radio unit (RRU), a radio frequency head (RH), a remote radio head (RRH), a relay, or a low power node, such as a picocell or a femtocell, or the like. In the context of the present disclosure, the terms "network device" and "base station" are used interchangeably for the sake of discussion, and the eNB mainly serves as an example of the network device. [0020] As used herein, the term "terminal device" or "user equipment" (UE) refers to any terminal devices capable of wireless communications with each other or with the base station. As an example, the terminal device may comprise a mobile terminal (MT), a subscriber station (SS), a portable subscriber station (PSS), a mobile station (MS) or an access terminal (AT), and the above devices mounted on a vehicle. In the context of the present disclosure, the terms "terminal device" and "user equipment" are used interchangeably for the sake of discussion. [0021] As used herein, the term "demodulation reference signal" (DMRS) refers to a pilot signal in signals transmitted by the terminal device to the network device over the PUSCH channel, which is used at the network device for channel estimation for the terminal device, for example. The DMRS may be located on a symbol of a resource block. [0022] As used herein, the term "includes" and its variants are to be read as open terms that mean "includes, but is not limited to". The term "based on" is to be read as "based at least in part on". The term "one embodiment" is to be read as "at least one embodiment". The term "a further embodiment" is to be read as "at least one further embodiment". Definitions related to other terms will be presented in the following description. [0023] As described above, in current 3GPP standardizations, for example, in LTE Release 14, the MU-MIMO technology has been proposed for the LTE. In the MU-MIMO, since a plurality of user equipment (UEs) may share bandwidths or physical resource blocks in uplink, for example, on the PUSCH channel, it is desired to multiplex the uplink signals from the plurality of UEs in an interleaved approach. [0024] When the uplink signals are multiplexed with such a technology as the IFDMA, the number of physical resource blocks (PRBs) allocated by the base station to the plurality of UEs in the LTE system has to be an integral multiple of the repetition factor (RPF, which is the number of UEs, for example), that is, has to be divisible by the repetition factor. For instance, in the case of the MU-MIMO, when the repetition factor is 2, an odd number of physical resource blocks cannot be allocated to the UEs. [0025] Through research of the inventor, it is found that this problem is resulted from design issues of the DMRS signals transmitted on the PUSCH channel. In order to at least in part solve the problem, embodiments of the present disclosure provide a novel designing and utilization solution of a DMRS sequence. [0026] Embodiments of the present disclosure provide a method for multiple input multiple output (MIMO) communications. The method comprises: receiving, at a terminal device, scheduling information from a network device, the scheduling information at least indicating physical resource blocks shared by the terminal device and at least one further terminal device, the number of physical resource blocks being indivisible by a total number of the terminal devices; generating a demodulation reference signal (DMRS), a length of the DMRS being determined based on the number of physical resource blocks and the number of at least one further terminal device; and transmitting the DMRS to the network device on sub-carriers allocated to the terminal device in the physical resource blocks. [0027] Fig. 1 shows an example communication network 100 in which embodiments of the present disclosure can be implemented. The communication network 100 comprises a network device 140 and a plurality of terminal devices, such as, a first terminal device 110 and a second terminal device 120. The network device 140 may communicate with the two terminal devices 110 and 120. Accordingly, the two terminal devices 110 and 120 may communicate with each other via the network device 140. It is to be understood that the numbers of network devices and terminal devices as shown in Fig. 1 is only for the purpose of illustration, without suggesting any limitations. The network 100 may comprise any suitable numbers of network devices and terminal devices. [0028] The communications in the network 100 may be implemented according to any suitable communication protocol, including but not limited to, cellular communication protocols of the first generation (1G), the second generation (2G), the third generation (3G), the fourth generation (4G), the fifth generation (5G), and the like, wireless local network communication protocols, such as Institute of Electrical and Electronic Engineers (IEEE) 802.11 and the like, and/or any other protocols currently known or to be developed in the future. Furthermore, the communications use any suitable wireless communication technology, including but not limited to, code division multiple access (CDMA), frequency division multiple access (FDMA), time division multiple access (TDMA), frequency division duplex (FDD), time division duplex (TDD), multiple input multiple output (MIMO) or multi-user multiple input multiple output (MU-MIMO), orthogonal frequency division multiple access (OFDMA), and/or any other technologies currently known or to be developed in the future. [0029] According to embodiments of the present disclosure, by using a newly designed DMRS sequence, when the terminal devices 110 and 120 multiplex the uplink signals transmitted by the terminal devices 110 and 120 with the IFDMA, the number of physical resource blocks used by the terminal devices 110 and 120 on the uplink PUSCH channel is more flexible. [0030] Principles and specific embodiments of the present disclosure will be described in details with reference to Figs. 2 and 3 from the perspective of the first terminal device 110 and the network device 140, respectively. Reference is first made to Fig. 2 which shows a flowchart of an example method 200 according to some embodiments of the present disclosure. It is to be understood that the method 200 can be implemented at the first terminal device 110 as shown in Fig. 1, and a method 300 can be implemented at the network device 140 as shown in Fig. 1. For the purpose of discussion, the method 200 will be described below with reference to Fig. 1. [0031] As shown in Fig. 2, at 205, scheduling information is received at the terminal device 110 from the network device 140. In one embodiment, in the MU-MIMO communications for the LTE, the network device 140 may transmit to the first terminal device 110 over a physical downlink control channel (PDCCH) the corresponding scheduling information for resource scheduling, for example. The scheduling information may be for example downlink control information (DCI). [0032] According to embodiments of the present disclosure, the scheduling information at least indicates physical resource blocks shared by the DMRS of the first terminal device 110 and a further DMRS of, for example, the second terminal device 120 (or other DMRSs of more terminal devices), and different sub-carriers allocated to the DMRS and the further DMRS in each of the physical resource blocks. The physical resource block may define time and frequencies available for the transmission of the DMRSs of the above terminal devices 110 and 120 in uplink over the PUSCH channel. Code division multiplexing of the DMRSs of the above terminal devices 110 and 120 may be performed using the same OCC codes. The number of physical resource blocks may be indivisible by a total number of the terminal device 110 and at least one further terminal device 120. In other words, the number of physical resource blocks is not an integral multiple of the total number of the terminal device 110 and at least one further terminal device 120. [0033] At 210, a demodulation reference signal (DMRS) is generated, and the length of the DMRS is determined based on the number of physical resource blocks and the number of at least one further terminal device 120. In one embodiment, in response to receiving the scheduling information, the terminal device 110 generates data to be transmitted on physical resource blocks or sub-carriers allocated to the DMRS of the terminal device on the PUSCH channel, for example. The data comprises a demodulation reference signal (DMRS), which is a pilot signal used for channel estimation at a network device side, for example. The DMRS may be transmitted on a particular symbol (for example, the fourth symbol) of the physical resource block. The DMRS from the terminal device 110 and the DMRS from the at least one further terminal device 120 may form an interleaved signal based on the allocation of the resources or sub-carriers by the network device 140 in the physical resource blocks. [0034] In some embodiments, the method 200 may also comprise: determining the number of sub-carriers associated with the physical resource blocks. Then, the length of the DMRS is determined to be proportional to the number of sub-carriers and the number of physical resource blocks and inversely proportional to the number of at least one further terminal device 120. [0035] Specifically, in one embodiment, in the case where the IFDMA is employed, the length of the interleaved signal is proportional to the number of the sub-carriers and the number of shared physical resource blocks, for example. Similar to the conventional DMRS, a length of the interleaved signal may be an integral multiple of the number of sub-carriers associated with one physical resource block, for example, 24. According to one embodiment, a sequence for the DMRS signals may be selected from a sequence table in LTE specifications. [0036] According to a conventional DMRS design, the sequence table in the LTE specifications specifies sequences with a length of 12 which is the smallest unit of a DMRS length. Based on the conventional sequence table in the LTE specifications, the length of the DMRS of the terminal 110 is equal to an integral multiple of the number of sub-carriers associated with one physical resource block, for example, 12, 24, and the like, and the length of the interleaved signal has to be an even multiple of the sub-carriers, accordingly. In this case, according to the above descriptions related to the resource scheduling, the number of resource blocks allocated to the terminal device and corresponding to the sub-carriers is an integral multiple of the number of sub-carriers associated with one physical resource block, accordingly. This forms limitations on the resource scheduling at a network device side and resource usage at a terminal device side. [0037] The DMRS design according to embodiments of the present disclosure may effectively solve the above problem. First, a case is considered where the repetition factor (or the number of terminal devices) is 2, which is, for example, the case where the two terminal devices 110 and 120 are present as in Fig. 1. The terminal devices 110 and 120 share one or more physical resource blocks. When one physical resource block is shared, the DMRS length for each of the terminal devices 110 and 120 is 6, for instance. When the number of shared physical resource blocks is an odd, such as 3, 5, 7, 9, and the like, the DMRS length for each of the terminal devices 110 and 120 is, for example, 18, 30, 42 and 54. For example, the DMRS length is determined by first multiplying an integral multiple of the number of sub-carriers associated with one physical resource block by the number of shared physical resource blocks and then being divided by the RPF, as shown in the following table: [0038] To this end, according to embodiments of the present disclosure, by taking a length-6 DMRS as an example, a "6-tone" DMRS sequence table employed in R13 NB-IoT WI specifications for Internet of things may be used, which includes fourteen length-6 DMRS sequences. For a further DMRS with a length of n (n>12), a new DMRS sequence may be generated for the IFDMA by concatenating a plurality of conventional DMRS sequences. According to embodiments of the present disclosure, the new DMRS sequence may be generated by concatenating a conventional Rel. 8 DMRS sequence and a 6-tone DMRS sequence. As an instance, a length-18 DMRS sequence is generated by concatenating a length-12 DMRS sequence and the above length-6 DMRS sequence. According to embodiments of the present disclosure, a DMRS sequence seed for the 6-tone DMRS sequence may be configured separately from a conventional DMRS virtual cell identifier (VCID). In order to determine the virtual cell identifier for generating the DMRS sequence, PI JSCH the virtual cell identifier "ID is configured separately for the conventional DMRS and the 6-tone DMRS on a layer higher than, for example, a physical layer. According to embodiments of the present disclosure, when a cyclic shift is applied to the DMRS, the cyclic shift may be independently applied to the conventional DMRS and the 6-tone DMRS. It is to note that a new 6-bit sequence may be designed for truncating. [0039] According to another embodiment, a new DMRS sequence may be generated by truncating various DMRS sequences. According to embodiments of the present disclosure, a DMRS sequence with a length of n+6 may be generated by truncating the conventional DMRS sequence. For example, regarding a group of 30 conventional DMRS sequences in the sequence table, from the sequences, for example, ^ )>—>'$( >, any six values, such as [0040] Some of the DMRS sequences generated by the truncation are WW'—'WQ-') or CP(1)!"-!