Trainable parameters for gates with multiple inputs.

[igres26]

The trainable flag for quantum gates could be improved for quantum gates with more than one input where one of the parameters should be fixed and the other trainable.

A different flag of each parameter would be a solution.

Something with symbols might also work but be slower in general.

More complex parameterization of gates and inputs can be handled by the user before adding them into the quantum circuit, but we might want to provide something to ease the process in the future.

[scarrazza]

@igres26 thanks for reporting. I believe we can have a trainable flag which accepts a boolean or a mask.