adding module lookup for building trackers

[clumsy]

Discussed this in #802

TL;DR this adds the ability to reference a module path instead of entry_point for tracker backend as a fallback when entry_point is not found, e.g.:

[torchx.tracker]
my_tracker = torchx.tracker.mlflow.create_tracker

[tracker:my_tracker]
...

Test plan:

• Added a unit test, the original functionality is there but as a fallback we also check if we can find tracker backend using module lookup.

[clumsy]

For your consideration, @kurman and @kiukchung.

[clumsy]

@kiukchung removed hydra usage, following entrypoint syntax. Please let me know if you have further comments.

[kiukchung]

Thanks for the PR (and removing the hydra dep). Could you update the docs as well?
Its all done via docstrings in torchx/tracker/__init__.py

You'll want to update the Tracker Setup and split out the rest of the docs into entry_point and module registrations.

You can preview the docs locally by:

$ cd docs
$ pip install -r requirements.txt
$ make clean html
$ python -m http.server -d build/0.7.0.dev0/html

Then opening up the docs page on https://localhost:8080

see docs-build CI action for details.

[clumsy]

Ok @kiukchung I addressed the comments, updated the documentation. Additionally I extracted the load_module functionality and tests to be separate from entrypoints. I didn't test the documentation rendering yet.

[clumsy]

I addressed an unrelated check failure due a broken link in Kubernetes docs. KPF seem to fail for an unrelated reason, @kiukchung.

[clumsy]

@kiukchung do I need to make furhter changes to this PR? I don't have conflicts when fetch/rebasing on top of origin/main locally.