Optimize non-empty directory warning check in model checkpoint callback

[jjenniferdai]

What does this PR do?

Along the lines of #9074, consolidate more fs calls from checkpoint callback. For non-empty directory warning:

• move to after __resolve_ckpt_dir
• rank zero only

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 internal minor changes/refactorings)

PR review

Anyone in the community is welcome to review the PR.
Before you start reviewing make sure you have read 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 🙃

[ananthsub]

note for reviewers:

This PR is an optimization when using remote filesystems with lightning. this warning is currently called on every rank, which creates duplicate, bursty outbound network calls. in reality, we only needed to check this on global rank zero (assuming we have a common dirpath across nodes).

when training with larger configurations (e.g. 16 nodes x 8 gpus), we ran into quota limitations for our jobs. this will benefit anyone saving checkpoints using remote filesystems such as S3/GCS/azure storage

[ananthsub]

LGTM!

[codecov]

Codecov Report

Merging #9615 (9353dd1) into master (45200fc) will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #9615    +/-   ##
=======================================
- Coverage      93%     89%    -4%     
=======================================
  Files         180     179     -1     
  Lines       15116   15308   +192     
=======================================
- Hits        14043   13587   -456     
- Misses       1073    1721   +648