[train] alias train.Checkpoint to new Checkpoint

[matthewdeng]

Why are these changes needed?

When using the new storage path, ray.train.Checkpoint will point to ray.train._checkpoint.Checkpoint instead of ray.air.Checkpoint.

RAY_AIR_NEW_PERSISTENCE_MODE=0 python -c "import ray.train; print(ray.train.Checkpoint)"
<class 'ray.air.checkpoint.Checkpoint'>

RAY_AIR_NEW_PERSISTENCE_MODE=1 python -c "import ray.train; print(ray.train.Checkpoint)"
<class 'ray.train._checkpoint.Checkpoint'>

nit: Will update __module__ in a future PR.

Related issue number

#38571

Checks

• 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 :(

[zhe-thoughts]

Train API cleanup