Feature Optional numba

[mauicv]

Todo:

• Tox env tests

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What is this

This work branches off feature/dep-management and will make numba optional dependency of alibi-detect.

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What this PR doesn't include:

1. The install options in the docs. I think it makes sense to do this in a later PR on the feature branch for all the optional deps all at once, once this branch has been merged in.
2. The install options in the README.md for same reason as 1.
3. The config-driven work in the release branch. This will be done in a separate PR in order to manage Release/Master branch complexity issues.
4. Make Licenses updates. This will be done in a PR on the feature branch.

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Notes:

• Should be merged after Feature Optional backends functionality #538,
  • check tests functionality is correct as well as config.py
• For some reason, this work caused some typing errors. I'm not sure why exactly as they didn't seem related. One of them I ignored with a comment (see Intermittent typing error in pytorch mmd.py #540) the others I fixed

[ascillitoe]

@mauicv re the typing errors, the pytorch/kernels.py and pytorch/context_aware.py ones only seem to occur with numpy>=1.23.0. numba 0.55.2 requires numpy<1.23 hence these only became apparent when you removed numba from the CI install env. Still good you've fixed them imo.

Re #540 I'm still looking into that one...