CLIAMS:1. A method for auto calibration of an electronic trip unit using a rating plug, for use in circuit breakers, said method comprising: selecting, using an interface of one or more device, a derating factor for said electronic trip unit; generating, using a hardware interface of said device, a voltage signal based on derating factor selected, thereby measuring (6), using a measuring means, said voltage signal to generate one or more voltage value; and transmitting, from said measuring means, said voltage value to said electronic trip unit; feeding, to a signaling conditioning module of said electronic trip unit, said voltage signal to generate an output signal, thereby sampling, using analog to digital convertor in a microcontroller of said electronic trip unit, said output signal for computing (8) a root mean square (RMS) value of said output signal; and computing (9) at least one calibration parameter based on said voltage signal and said output signal, wherein said parameter is at least one of gain or DC offset; determining (10), using a firmware in said electronic trip unit, a derating factor of said electronic trip unit on the basis of said voltage signal; checking (11), using said firmware, a range of said at least one calibration parameter on the basis of said derating factor determined, thereby, if said calibration parameter is in range, multiplying (15) said gain with said derating factor; and storing (14), in a memory of said firmware, said calibration parameter. 2. The method as claimed in claim 1 comprises of receiving, by said microcontroller of said electronic trip unit, a power supply generated by a hardware interface. 3. The method as claimed in claim 1 and 2, wherein said at least one calibration parameter is at least one of a gain, or a DC offset, or any combination thereof. 4. The method as claimed in any of the preceding claims, wherein said gain is a ratio of said output signal in the numerator and said voltage signal in the denominator, and said DC offset is a DC shift given to voltage signal is obtained by dividing sum of samples values for a cycle by the number of samples for that cycle. 5. The method as claimed in any of the preceding claims, wherein said measuring means is configured to measure a voltage or a current or resistance or any combination thereof, and is selected from a group comprising a multimeter, a voltmeter, a ohmmeter, an avometer. 6. The method as claimed in any of the preceding claims, wherein said electronic trip unit is configured to achieve different gain for different de-rating values. 7. The method as claimed in any of the preceding claims, wherein said range of said gain is within the ±10% of the ideal value, and said range of said DC Offset is within the ±2% of the ideal value. 8. The method as claimed in any of the preceding claims, wherein said rating plug is implemented by modification in the gain of signal conditioning module. 9. The method as claimed in any of the preceding claims, wherein said calibration parameters range is selected in such a manner that a minimum values and a maximum values covers entire de-rating range, and hence a protection firmware and a metering firmware is unaltered for electronic trip units with different de-rating factor. 10. A system for auto calibration of an electronic trip unit, for use in circuit breakers, said system comprising: one or more device(s) comprising one or more interface(s) and one or more hardware interface(s), configured to: enable selection of a derating factor for said electronic trip unit using said interface; and generate a voltage signal based on said derating factor selected using said hardware interface, thereby intercommunicating said hardware interface and one or more interface(s); transmit said voltage signal to one or more measuring mean; said one or more measuring means configured to: measure said voltage signal received to generate one or more voltage value; and transmit said voltage value to said electronic trip unit to said electronic trip unit; said electronic trip unit comprising one or more signaling conditioning module, one or more microcontroller having an analog to digital convertor, and one or more firmware, configured to: sample said output signal using said analog to digital convertor for computing a root mean square (RMS) value of said output signal; compute at least one calibration parameter based on said voltage signal and said output signal, wherein said parameter is at least one of gain or DC offset; determine a derating factor of said electronic trip unit on the basis of said voltage signal using said firmware; check a range of said at least one calibration parameter on the basis of said derating factor determined using said firmware, thereby, multiply said gain with said derating factor, if said calibration parameter is in range; and store said calibration parameter in a memory of said firmware. 11. The system as claimed in claim 4, wherein said device, said measuring means, and said electronic trip unit are communicable coupled to each other and configured to transmit and receive data. 12. The system as claimed in any of the preceding claims, wherein said signal signaling conditioning module samples of said output signal using a microcontroller in said electronic trip unit. 13. The system as claimed in any of the preceding claims, wherein said rating plug is implemented by modification in the gain of signal conditioning module. 14. The system as claimed in any of the preceding claims, wherein said calibration parameters range is selected in such a manner that a minimum values and a maximum values covers entire de-rating range, and hence a protection firmware and a metering firmware is unaltered for electronic trip units with different de-rating factor. ,TagSPECI:TECHNICAL FIELD The present subject matter described herein, in general relates to circuit breakers, and more particularly, to an auto calibration of electronic trip unit of a circuit breaker. BACKGROUND The typical implementation of circuit breaker using electronic trip unit for single phase or three phase involves usage of current sensors. The current is passed through a primary winding and very low current output is available at the secondary. This current value is given to electronic trip unit for processing. The ratio of primary current to secondary current is N1:N2 where N2<