Improve typing

[mrossinek]

What should we add?

While working on #73, @ElePT and I have identified the following inconsistencies and/or problems around the type hints which should be investigated further. Rather than holding up that PR unnecessarily, we decided to gather these here.
That way, they can be investigated one-by-one.

Note: some of these may very likely require code changes to resolve inconsistencies in the interfaces.

• Typing around np.ndarray vs Sequence[float]. It is a known issue that to mypy the numpy arrays do not satisfy the Sequence type. This leads to many type ignore statements right now which can hopefully be improved upon.
  • For example, typing around parameter values being passed on to the primitives: sometimes its Sequence[float], sometimes its Sequence[Sequence[float]], and oftentimes internally it ends up as np.ndarray causing many type ignore comments
• The typing of the initial_point attributes is very inconsistent across its various occurrences.
• The Minimizer interface does not appear to be supported properly by VQD and PVQD (for example) since they do not provide the required jac argument
• The ListOrDict type is causing quite some oddities. This has multiple cases:
  • for the metadata (e.g. variance) types and implementations are inconsistent with the variance sometimes wrapped in dictionaries and sometimes provided as a single number
  • for aux_operators and/or observables we had to add many type ignores which would need to be re-evaluated (possibly linked to the previous point)
• All classes that use the EsimationProblem have very poor typing around the following points:
  • the post_processing function
  • the confidence intervals
  • the circuit_results
• Add py.typed file back again - removed by Remove py.typed until #74 is addressed #121

[woodsp-ibm]

The Estimation logic - and possible elsewhere - has code that is no longer used. While deprecated code for QI/opflow was removed there are some util like functions taking types that would have been an outcome - this code is now untested.

Also Grover has some issue around using the circuit passed a as grover operator - it allows QuatumCircuit but then treats it as GroverOperator where it unconditionally accesses an attribute that has which is not present on a QuantumCircuit. Update: I raised #76 for this.

This was just from starting to look at mypy in AA and AE folders.

[mrossinek]

I suppose a good strategy at finding which of the code is unused would be to look at the coverage reports. Obviously not all uncovered lines can be removed but it might be a good starting point 👍

[woodsp-ibm]

Yes, particularly around the algos AE, Grover and Phase Estimation that were modified to add Sampler logic paths to the existing code and whether more could/should have been removed when QI paths were removed that was part of common utilities.