Add AOT tools

[riga]

I'm opening this PR for @Bogdan-Wiederspan who did the actual code prototyping and implementation in this PR. However, we needed access to the CI pipelines, which is why the feature branch is not part of his fork but already in the main repo.

---

This PR adds all tools necessary

• to inspect graphs regarding their AOT/XLA compatibility,
• to specialize signatures of SavedModels (reading a graph with batching support from a SavedModel and converting it to one or more graphs that each have a fixed batch size), and
• to AOT compile this specialized graphs.

New unit tests are provided along with docstrings for all new helpers.

Besides these additions, a few minor changes to existing code are provided, mostly to align the existing tensorflow tools to python 3.

Once merged, we should

• release a new tag, perhaps 0.2.0,
• publish to pypi, and
• update the main cmsml documentation.

@valsdav

[valsdav]

Hi @riga @Bogdan-Wiederspan , thanks a lot for the PR and for including all the tests and documetation.

I can tag the repository, but I guess you have the credentials for the update to pypi, right?

[valsdav]

:rocket: