Fix QAOA

[t-imamichi]

Summary

Fix QAOA of opflow.

Details and comments

• Allow users to change the number of qubits of QAOA
  • Change the order of check and varform initialization
• Track appropriate parameters associated with QAOA var form
  • Add a check of number of qubits

[t-imamichi]

I found that this change breaks some unit tests of chemistry, e.g., test/chemistry/test_uccsd_hartree_fock.py
I need to check the details.

Update: I fixed this problem. See my comment below.

[t-imamichi]

I found out the root cause. QAOA._check_operator generates new parameters with same label, which have different hash value. I fix it by checking number of qubits as VQE._check_operator_varform does. We need the check in both VQE and QAOA because they are both invoked.

[woodsp-ibm]

Ok. At first sight it looks like the order of the check_operator and creation of var form were just swapped with the added qubit check that you would think was for efficiency to prevent building out the var form again, and if you did it would not be an issue. The check though is in fact needed because of this parameter sorting.

With the code change on the move from Aqua, which removed the operator from constructor and the setter, meaning it can only be passed on the compute_min_eigval, I think going forwards there is room to improve refactor/improve the logic around this checking/building etc. Especially if the parameters end up with a defined ordering #5614