Operators should have a Backend

Since Operator objects are linked to forward() and backward() kernels on specific devices, they should have a Backend parameter.

This parameter could be very useful to compare Operators' backends for Operator copy constructor !8 (merged) or inserting Transmitters. Indeed for now, to know the backend of an Operator, one should look at the backend of the implementation of the output Tensor of the Operator (if the implementation exists!)