[Train V2] Hide the private functions of train context to avoid abuse

[hongpeng-guo]

Why are these changes needed?

In V2, users should not be able to access the private methods / attributes.

For example, ray.train.get_context()._save_checkpoint should not be accessible. Only the public APIs (get_world_rank, etc.) should be available from code completion.

Added a TrainContext class in the ray/train/v2/api/context.py

Related issue number

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• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[justinvyu]

Thanks!

[hongpeng-guo]

Updates: comments are handled. PTAL @justinvyu

[hongpeng-guo]

Take one more look ? @justinvyu

[justinvyu]

Just some remaining nits. should be good after this!