CustomSurrogate class and custom_surrogate function added to the surrogate.py file.

This enables the users to define their own custom surrogate function and call it using either
custom_surrogate(name_of_custom_surrogate_function)
or
CustomSurrogate.apply(data, name_of_custom_surrogate_function)

The arguments of the custom surrogate gradient function always are The input of the forward pass (input_), the gradient of the input (grad_input) and the output of the the forward pass (out) respectively.

** Important Note: The hyperparameters of the custom Surrogate gradient function have to be defined inside of the function itself. **