Introduce CheckpointIO Plugin

[SeanNaren]

What does this PR do?

Closes #8118.

This is a V0 where we've abstracted the checkpointing logic into a class that eventually can be plugged in via the trainer.

Few outstanding issues that I'll tag throughout the PR!

The goal is to be able to do this:

trainer = Trainer(DDPPlugin(checkpoint_plugin=MyCustomCheckpointIO()))

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[pep8speaks]

Hello @SeanNaren! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-08-13 11:28:41 UTC

[awaelchli]

I like this idea <3

[codecov]

Codecov Report

Merging #8743 (8f234e0) into master (b47e3ab) will increase coverage by 0%.
The diff coverage is 91%.

@@          Coverage Diff           @@
##           master   #8743   +/-   ##
======================================
  Coverage      93%     93%           
======================================
  Files         169     172    +3     
  Lines       14061   14148   +87     
======================================
+ Hits        13033   13118   +85     
- Misses       1028    1030    +2     

[SeanNaren]

My plan:

1. Ensure all the tests pass
2. Add tests for checkpoint plugin as stands
3. Investigate if we can rely on the checkpoint plugin to run the rm that happens in the ModelCheckpoint callback, and some of the other fspec calls

[awaelchli]

@ananthsub Maybe marking it experimental in changelog + docstring of plugin class?

[awaelchli]

thanks @SeanNaren
looks like a great addition

[SeanNaren]

general question: though this is brand new, how should we communicate that it's experimental and the API might change before the next release? asking for developers who work off master

as @awaelchli suggested I'll add some docstrings, however I also think this should go into the docs just like other features we have for discoverability. Would people be against this?

[ananthsub]

awesome work @SeanNaren !

[ananthsub]

@bowangbj - the collective approach for #7534 should end up looking pretty similar to this i think

[carmocca]

Awesome!