generate operator implementation templates

[nopeslide]

Just thought that we could also generate the files included inside src/operators/implementation (i.e. operator__onnx__relu__6__T_tensor_float.c) from the Python script. So we can avoid manually creating them. Maybe all types is too much by now, so I would create only the float variants.

Quite useful also if someone wants to implement an operator. If the file and function is already in place (but empty) would be easier for someone without much knowledge of the code to implement and operator. We can abstract them from the complexity of running the Python script and so on.

Originally posted by @alrevuelta in #18 (comment)