[Train] Update PyTorch Lightning import path

[woshiyyya]

Why are these changes needed?

PyTorch Lightning 2.0 updated the import path from pytorch_lightning to lightning.pytorch. This change will first detect if the latest import path exists, then fallback to the original one.

Related issue number

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 :(

[matthewdeng]

Awesome ⚡

[woshiyyya]

Rerun the release tests that are using lightning 2.0

✅ dolly v2: https://buildkite.com/ray-project/release-tests-pr/builds/55087#018b02a5-bc24-4210-bf63-2ab4c6ed2907
✅ vicuna: https://buildkite.com/ray-project/release-tests-pr/builds/55140#018b070b-42e5-484f-a300-83256a4df5e9

[woshiyyya]

Ok. Both release tests passed!

Resolved the merge conflicts and run the full CI again.