Add backward functions for ReLU, Sigmoid and Tanh

Context

Add some backward functions that were missing.

Detailed modifications

• Use a protype for backward function that may fit all the needs : input, output and grad_output are needed to compute grad_input in the general case (input was missing, output is kept to save computation). This may lead to unused parameters, but it is probably not an issue ?

• Add missing backward functions for Sigmoid and Tanh.

• Use input instead of output for testing the sign in ReLU (both codes should give same results, but the use of input is preferred).

TODO

For this merge request :

• Fell free to adapt the proposed modifications to match code requirements.
• Check if my understanding of the way the forward methods should work is correct.

For future :

• Check the forward function for Sqrt operator that seems not to be correct.
• Homogenize the way forward and backward are coded for all available operators.