Activation function improvements

[Jaswar]

Some activation functions can be improved by using Tensor operations (instead of designated methods in the form of evaluators).

This is especially true for sigmoid and linear activation functions where both the forward and backward steps can be defined with tensor operations (ReLU would require a definition of 2 new tensor operations).

This will likely cause the activation functions to use more auxiliary space, so maybe some kind of basic memory manager could be used (that would contain some working spaces that are shared by activation functions/losses).