Add support for reloading the last checkpoint saved by passing ckpt_path="last"

[otaj]

What does this PR do?

Implements #11912

Allows you to pass keyword "last" to ckpt_path argument. "best" still remains the default value if None is passed, except one particular case - in method fit when fault tolerant training is enabled.

Value last selects the checkpoint with the latest modified timestamp from set of all last checkpoints (currently only one, because they overwrite each other, but once #5030 is solved, it could be a different one) and all fault tolerant checkpoints. If no checkpoint in these two sets is found (due to fault tolerant training not being enabled and passing save_last=False to ModelCheckpoint), warning is issued and no checkpoint is loaded.

What needs to be done before making it non-draft

• Write tests
• Add changelog entry
• Update documentation
  • is the change of docs big enough?
• Discuss whether this design makes sense

Does your PR introduce any breaking changes? If yes, please list them.

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

PR review

Anyone in the community is welcome to review the PR.
Before you start reviewing, make sure you have read the review guidelines. In short, see the following bullet-list:

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified

Did you have fun?

Make sure you had fun coding 🙃

cc @Borda @awaelchli @ananthsub @ninginthecloud @rohitgr7 @carmocca @justusschock @jjenniferdai

[carmocca]

LGTM! I've pushed some minor simplification to the code and greater simplifications to the test to narrow their scope to just these warnings.

[rohitgr7]

great!