Added default operator impl with default producer-consumer model

This update simplifies a lot the implementations, by providing a default producer-consumer model to the base OperatorImpl class!

The change is relatively minor and should not jeopardize potential future refactoring around operators modeling.

For the default producer-consumer model, the assertion made is simple: the full input tensors are consumed and the full output tensors are produced at each forward execution. Futhermore, input tensors cannot be overwritten during execution (the operation is not in-place).