DESC:FIELD OF THE INVENTION: The present disclosure relates to an anode assembly for aluminium electrolysis cells used for aluminium production. More particularly, the present disclosure relates to a reduced voltage drop rodded stepped stub anode assembly for an aluminium electrolytic cell. BACKGROUND OF THE INVENTION: Aluminium has turned out to be the wonder metal of the industrialized world. No other single metal is so versatile in use and economics. Aluminium’s growth rate is the highest amongst the major basic metals today. However, the aluminium production is highly energy consuming. The major cost components of aluminium production are DC Power, Alumina and Carbon anode. Aluminium is conventionally produced by the Hall-Heroult process in which alumina is dissolved in molten cryolite and electrolyzed with an intense direct current (DC). The cryolite bath is contained in electrolytic cells lined with carbon cathodes while carbon anodes hang from rods connected to an electrical bus bar. The alumina is reduced with oxygen being deposited on the anodes and forming mostly carbon dioxide while the molten aluminium is deposited on the cell bottom and periodically tapped. The reduction process is performed in a large aluminium reduction cell which includes a container or "pot" lined with refractory and carbon. Within the pot is a molten mixture of alumina dissolved in cryolite and other materials, such as various fluorides, which are generally referred to as "bath". The pot and a metal pad of molten aluminium, which collects at the bottom of the pot, form an associated cathode. A voltage potential is applied between the carbon anodes and the pot, resulting in a large current flow from the anodes, through the molten bath mixture, to the pot. The electrical current passing through the bath mixture reduces the alumina into its aluminium and oxygen components. The aluminium drops to the bottom of the pot, forming the metal pad. The oxygen combines with the carbon from the anodes and escapes as carbon dioxide gas, which is vented from the pot. As the alumina is consumed, more alumina is added to the bath in the cell. During the aluminium reduction process, the carbon anodes are consumed, therefore, the spent anodes are routinely replaced by fresh anodes so that the aluminium production can continue in an efficient manner. The decomposition voltage of alumina to aluminium is 1.60 V but the normal voltage required for smooth pot operation is 4.40 V. This excess power consumption during smelting is due to various drops:- Anode rod to stub drop – Depends on welded joint quality, Stub to carbon drop – Surface area and contact quality between stub and Carbon block, Cathode lining drop - Cathode construction, Clamp Drop - Quality of surface contact & tightening of Clamps, Fixed drops – Bus Bar dimensions & current density, Electrolyte Drop – Depend on electrolyte chemistry. Higher resistance at stub-carbon interface leading to heat generation, restricting increase of pot line current thereby limiting the avenues of productivity / efficiency improvement. The anode connection has to be replaceable, tight and resistance to deterioration. A loose connection causes high voltage drops, while a cramped connection causes crack in the anode block. Moreover, the aggressive environment of the aluminium production cell causes continuous stub deterioration and stub bending. The known prior arts of anode assembly have the drawbacks of high voltage drop at stub-anode connection and have high power consumption and involve high cost. The non-patent disclosure titled as “Low resistance anode assembly using steel stubhole conductors across the cast iron to carbon interface (Edward Williams, W. Berends, S. Haley and M. Gagnon) relates to anode assemblies which suffer a significant electrical contact resistance across the cast iron to carbon interface that is inversely dependent on contact pressure and area. Industry efforts have incrementally reduced this electrical resistance by increasing stub diameter, changing iron chemistry and by improving the stubhole shape. The additional use of multiple steel conductors to bridge across the cast iron to carbon interface provides a means to further reduce the electrical resistance. The function of the conductors is independent of the iron to carbon contact pressure, the stub temperature, iron chemistry, and the stubhole shape. The steel conductors are tightly driven into the carbon anode at one end, with the other end bonded into the cast iron. This paper includes in-pot performance testing results which demonstrate the reduced resistance when using stubhole conductors. However, in this disclosure nails (conductor) are provided with the help of pneumatic tools in the anode carbon block & current is passed through these multiple nails to avoid the air-gap between carbon block & cast iron. WO2016/141475 titled “anode assembly for aluminium electrolysis cells and method for manufacturing anode assemblies” discloses an anode assembly for an aluminium electrolysis cell. The anode assembly includes a baked anode block, a plurality of elongated connection elements each having an anode block contact surface and an electrical connection surface, at least one electromechanical crossbar connector covering the electrical connection surfaces of the elongated connection elements, and a crossbar electrically connected to the elongated connection elements. The method of manufacturing an anode assembly includes the steps of forming a block of green anode paste, inserting a plurality of elongated connection elements in the green anode paste, baking the green anode, positioning a crossbar above the electrical connection surfaces of the plurality of elongated connection elements, and covering the electrical connection surfaces and at least partially the crossbar with a surface-conforming electrically-conductive material. However, during green anode manufacturing of carbon block, the conductor have been forged with plurality of elongated connection elements and stub has not been used. Also, the elongated structure has to be baked along with in baking furnace for strengthening. EP2006419 titled “reduced voltage drop anode assembly for aluminium electrolysis cell” discloses an anode assembly for aluminium electrolysis cells comprising carbon anodes with stubholes and an anode hanger having stubs, whereas the anodes are fixed to the anode hanger by cast iron, characterized in that the stubholes are fully or partially lined with an expanded graphite lining. Further, mechanical stresses in the stubhole area are reduced. By further forming a collar from the lining, the spilling of cast iron over the anode surface is prevented and optionally a protective shot plug or a protective collar prevent direct contact of the hot electrolyte bath with the stub and the cast iron. However, this prior art discloses the graphite lining in which baked anode block is coated with graphite to reduce the voltage drop. WO 2015089654 titled “Low resistance electrode assemblies for production of metals” discloses an electrode assembly for use in a reduction cell for the production of metal such as aluminium. The electrode comprises an electrically conductive carbon electrode block with an electrically conductive metal member connected thereto. The connection provides an improved electrically conductive connection between the carbon electrode block and the conductive metal member, with reduced resistance. The insert may provide a direct connection between the electrode block and the metal member, or the connection may be provided through a layer of cast iron or other metal element provided between the electrode block and the metal member. Generally, in conventional aluminium smelter electrical connection is done by copper / aluminium rod with stub and in Rodding Shop molten cast iron is poured in the hole / carbon block through which current passed to the Pot. Therefore, in smelter, high Stub to Carbon drop has an impact on specific power consumption, cost of aluminium production and life of Anode rod assembly. OBJECTIVES OF THE INVENTION: An objective of the present disclosure is to reduce the voltage drop at the anode connection by increasing contact surface area between anode block and stub. Another objective of the present disclosure is to reduce stub to carbon drop in a smelter. Still another objective of the present disclosure is to reduce heat dissipation through the stub. Still another objective of the present disclosure is to reduce the cost of aluminium production. SUMMARY OF THE INVENTION: An aspect of the present disclosure is to provide a rodded anode assembly for an aluminium electrolytic cell, the anode assembly comprising:- an anode block, the anode block having a stubhole; a stub having a first end portion and a second end portion, the first end portion of the stub accommodated in the stubhole in the anode block; a copper bar electrically and mechanically connected through a stiffener by welded joint to the second end portion of the stub; Characterized in that the first end portion of the stub has an enlarged diameter and the second end portion of the stub has a reduced diameter, the first end portion abutting the second end portion; the dimensions of the first end portion and second end portion being configured such that the first end portion accommodated in the stub hole has maximized surface area thereby reducing the voltage drop between the anode and the first end portion of the stub. Another embodiment of the present disclosure provides that the stub is made of mild steel. Still another embodiment of the present disclosure provides that the anode block is made of carbon. Still another embodiment of the present disclosure provides that clearance between the stubhole and the first end portion of the stub has cast iron filling forming a thimble around the first end portion of the stub. Still another embodiment of the present disclosure provides that dimensions of the stubhole are configured in proportion to dimensions of the first end portion of the stub. Another embodiment of the present disclosure provides that the stub is mechanically, thermally and electrically connected to the anode block by casting molten iron. Another embodiment of the present disclosure provides that an elongated copper bar is connected to the stub through stiffener made of mild steel and it helps in penetrating copper bar in mild steel stub to strengthen the weld joint. These and other features, aspects and advantages of the present subject matter will become better understood with reference to the following description and appended claims. This summary is provided to introduce a selection of concepts in a simplified form. This summary is not intended to be used to limit the scope of the claimed subject matter. BRIEF DESCRIPTION OF DRAWINGS: Figure 1 shows rodded anode assembly with stepped stub for an aluminium electrolytic cell (pot cell); Figure 2 shows stepped Stub anode assembly in the electrolytic cells (pot cells) of a smelter; Figure 3 shows the stepped stub accommodated in stubhole; Figure 4 (a) shows the stepped stub according to the present disclosure; Figure 4 (b) shows normal stub as per the prior arts; Figure 5 (a) shows graph for voltage of an electrolytic cell (pot cell) with stepped stub; Figure 5(b) shows graph for voltage of an electrolytic cell (pot cell) with normal stub. DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS: Industrial production of aluminium has very high energy consumption, therefore, improvements in energy efficiency can be made by voltage saving at the anode connection. The voltage drop between stub and carbon anode is an essential part of the overall voltage drop at the anode and has a detrimental impact on the electrolytic process. Smelter Power consumption accounts for approx. 40% of the cost of aluminium production thus, any saving in Power consumption has high impact on the cost of aluminium production. Primary aluminium is produced by electrolytic reduction of alumina, the process is commonly known as Hall-Heroult Process. alumina is dissolved in molten cryolite bath and electrolysis is carried out in specially designed aluminium electrolysis cells commonly known as 'pots'. An electrolytic cell consists of the following components:- Anode (the +ve terminal) – Prebaked, made of CP coke and coal tar pitch; Cathode (the –ve terminal) – made of cathode carbon blocks in which steel collector bars are embedded to make electrical connection; Electrolyte – Medium for carrying out electrolysis, consisting of mainly, cryolyte, aluminium fluoride and calcium fluoride in which alumina is dissolved for electrolysis; Electrolytic cells are connected in series along with certain bus bar configuration for passage of current constitutes a potline, spread over in two rooms. In an electrolytic cell, when DC current is passed from anode to cathode through molten electrolyte, alumina dissolved in electrolyte is dissociated into aluminium & oxygen. Aluminium being positively charged goes to the cathode while oxygen being negatively charged goes to anode. aluminium continues to deposit at cathode, which is tapped out periodically. Oxygen reacts with anode carbon forming CO2 & CO, which ultimately goes out of cell as pot gases. The consumed anodes are replaced in certain sequence periodically with new anode to continue the process. Reduction Plant, in particular comprises of Carbon Plant, and Pot rooms. In Carbon Plant, carbon electrodes (anodes and cathode Paste) are produced which are utilized during electrolytic process of reducing alumina powder to molten aluminium metal. The metal produced in pot rooms is finally converted into value added products viz. Properzi wire rods, Slabs, Billets, ingots. Further value on these products is added in Fabrication Plant. Anode Manufacturing for Aluminium Production:- Carbon anode is one of the major raw material in aluminium smelter operation. It takes active part in electrolysis and gets consumed in the process. On completion of its scheduled stay in pots, the anode is replaced by a new one and the remaining portion known as butt is processed together with calcined coke and Binder Pitch for fresh anode production. The carbon anode is manufactured in three stages: GREEN ANODE The ingredients in anode manufacturing are Calcined Petroleum Coke (C.P.coke), High softening point Coal Tar Pitch and Pot room returned butts. C.P. Coke consignment received from various sources is stored in coke silos. For the production of anodes, coke is passed through primary and secondary crushers and screened. Potroom return butts are fed to jaw crushers and crushed in 4”-6” sizes. The same is passed through hammer mill and crushed into -3/4” particle size. This material is stored in butt silo and transferred to bins as butt fraction. Pitch is stored holding tanks from where it is transferred to buffer tank then weighing scale for consumption. Dry aggregate constituting 5 fractions coarse, medium, fine, BMP and filter dust fractions are separately weighed, as per the standards specified, and charged into preheater. The dry aggregate is mixed and heated up in preheater and then transferred to buss mixer where measured quantity of molten pitch is added to the mixer and prepared paste. Hot paste is then transferred to surge hopper. From surge hopper, the weighed quantity of paste is feed to transfer trolley which finally feeds to vibroforming mould and form anode subsequently into green anodes. Formed anodes are water cooled and unloaded on skips (carrier for carrying green anodes). These anodes are checked for their physical appearance, height, weight & density and only good anodes are transferred for baking. BAKED ANODE The good green anodes from paste plant are packed in empty pits of Baking Furnace. All the anodes are covered by C.P. coke all around. The top of the last layer anode is covered with C.P. Coke as blanket material. These anodes are baked at a temperature around ~1100 deg. C (min) in the pits and the total baking cycle takes about 15-20 days. During the baking process the binder pitch gets converted into pitch coke. The released volatile matter contributes in baking of anodes. The firing system in baking furnace consists of computerized oil (F/Oil) fired burners regulated by temperature feedback of flue thermocouples. The baking process improves anode quality interns of strength, reactivity, thermal and electrical conductivity etc. On completion of baking process and cooling, anodes are unpacked from the sections and cleaned properly. Only good anodes are loaded on conveyor belt for further cleaning and subsequent rodding. RODDED ANODE Baked anodes are fixed with Cu-steel stub assembly by pouring molten cast iron around the steel stub, in the anode hole. On solidification the Cu - stub assembly gets fixed with the carbon anode. A reference mark is marked at about 2.2 meters from the bottom of the anode, on copper bar. The ready anode is then sent to Potroom for replacing the old consumed anodes in pots. The rodding shop has the induction furnaces where the cast iron charge is prepared. A rodded assembly with anode block comprises of copper bar and stub welded in house the length of the individual material i.e. Cu bar stub and an anode block may varies from one assembly to another therefore the marka (painted line) is marked from bottom reference these marka line helps for purposed of anode setting. In the aluminium smelter factors like cost of stub material, weight of stub material, cast iron quantity, heat Loss, weld strength depend on the stub size. Stub to carbon drop has significant contribution in overall power consumption and depends on Stub dimension, Cast Iron quality, Stubhole design and Pouring Parameters. The present disclosure provides a rodded anode assembly with stepped stub for an aluminium electrolytic cell wherein stub is designed in such a fashion that the contact surface area inside the hole is kept maximum to reduce the stub to carbon drop for anode thereby minimizing the cost involved. Referring to figure 1, which shows rodded anode assembly with stepped stub. The rodded anode assembly (1) comprising a carbon anode block (2), an electrically conductive stub (4) having a first end portion (A1) and second end portion (A2), the first end portion (A1) of the stub (4) is accommodated in the stubhole (3) of the anode block (2), a copper bar (5) electrically and mechanically connected through stiffener by welded joint to the second end portion (A2) of the stub. The clearance between the stubhole (3) and the first end portion (A1) of the stub has cast iron filling (6) forming a thimble around the first end portion (A1) of the stub. Referring to figure 2, which shows the stepped Stub anode assembly in pot cells of the smelter. Various components of the assembly are described as below:- • Anode block (2) - It is a consumable material comprising of calcined petroleum coke and hard pitch. When current passes through it to perform electrolysis reaction it gets consumed. • Stepped Stub (4) – it is made of mild steel and conduct electric current from copper bar to Anode Block through cast iron. • Copper bar (5) - It is a rectangular section of copper for conducting electric current anode ring bus to carbon block. Figure 3 shows the stepped stub accommodated in a stubhole and it is a cross sectional view of anode hole with stepped stub. Referring to figure 4(a), which shows the stepped stub designed in such manner that total length remains constant in comparison to normal stub shown in figure 4(b), however, total cross sectional area of the stub is increased i.e. (A< A1+A2), (D2