[Train] Deprecate LightningTrainer

[woshiyyya]

Why are these changes needed?

Deprecate LightningTrainer, uses should use TorchTrainer only in Ray 2.8.

We keep the code but immediately raise an Error when the users initialize a LightningTrainer, LightningCheckpoint, LightningPredictor. In 2.9, we will completely remove the code.

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 :(

[aslonnie]

not sure why this needs ci team to review.

[woshiyyya]

@aslonnie Seems that using git push -f will automatically tag ray-project/ray-ci for review?

[aslonnie]

Seems that using git push -f will automatically tag ray-project/ray-ci for review?

that should not be the case.. no idea..

you can try if you can reproduce it.

it is more likely that there was a messy rebase or something.