Make it possible to obtain the expressions used to convert qiskit parameters to quimb parameters

[garrison]

There is something potentially problematic with the way qiskit-addon-aqc-tensor uses qiskit-quimb. Currently, it is assumed that the parameter of a Qiskit gate matches the parameter passed to the quimb.tensor.Gate constructor. However, it is not guaranteed this is always the case. For instance, there might be multiplicative factors or offsets:

qiskit-quimb/src/qiskit_quimb/gate.py

Line 188 in 5eeccc7

"GIVENS2", params=[0.5 * theta, beta + 0.5 * math.pi], qubits=qubits, **kwargs

This specific line is likely to no longer exhibit this problem in a future release. However, it is always possible that a future version of qiskit-quimb might add a different gate which does not pass the parameter exactly, and if this happens, qiskit-addon-aqc-tensor will not be able to reason about the gradient with respect to such parameters correctly.

In general, if there are n parameters for a gate, then the map between qiskit parameters and quimb parameters is given by a n-by-n constant matrix for any given gate. It should be possible for a user to query this matrix. It'd be even better if it is somehow specified only once (DRY philosophy), or if the test cases are able to ensure that each one is correctly specified.