[Tune] Rename overwrite_trainable argument in Tuner restore to trainable

[justinvyu]

Why are these changes needed?

This deprecates the overwrite_trainable param and renames it to trainable, moving toward the goal state of looking like Tuner.restore(path, trainable=trainable, param_space=param_space). This also emits a warning if a trainable is not specified, as this will become a required argument starting in 2.5.

Context

The reason why trainable will become a required argument:

1. User has access to their trainable, since restore will not be running independently but alongside the original script.
2. There is currently special case warnings for with_parameters needing the trainable to be re-specified. Always requiring this argument would just simplify what the user needs to think about (when is it required, when is it not?).
3. There are other cases where the trainable could hold object references that we don't catch. Requiring the parameter would prevent users from falling into this and getting confused by the errors.

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 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 :(

[krfricke]

Looks like test_restore_with_new_trainer is failing: https://buildkite.com/ray-project/oss-ci-build-pr/builds/10783#0186043e-33ae-48dd-bf41-517d33b1e808

[justinvyu]

Fixed the test assertion to check for all warnings. Will ping again when tests are passing!