DeepSpeed ZeRO Update

[tchaton]

What does this PR do?

Adds DeepSpeed ZeRO 3 to the DeepSpeed plugin. Due to the nature of the plugin, we've had to make a few changes summarized below.

• Plugin has to manage saving logic as DeepSpeed only supports non-elastic checkpointing; i.e if you train on 8 GPUs, you must load the model on 8 GPUs. This will be addressed over the coming weeks as PRs are merged into DeepSpeed
• We've introduced a sharded context manager to create layers and shard them instantly.
• DeepSpeed in ZeRO 3 has to wrap the model with optimizers even in inference. This is a limitation of DeepSpeed where the sharding logic is tied to the Optimizer, hence we only get proper sharding logic when an optimizer is passed through. Overall this means we have to allocate some memory to optimizer state (which is already done within training).

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 update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

Anyone in the community is free to review the PR once the tests have passed.
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 🙃

[pep8speaks]

Hello @tchaton! Thanks for updating this PR.

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

Comment last updated at 2021-03-30 16:19:26 UTC

[codecov]

Codecov Report

Merging #6546 (83e1343) into master (3c86193) will decrease coverage by 6%.
The diff coverage is 43%.

@@           Coverage Diff           @@
##           master   #6546    +/-   ##
=======================================
- Coverage      91%     86%    -6%     
=======================================
  Files         192     192            
  Lines       12231   12494   +263     
=======================================
- Hits        11184   10710   -474     
- Misses       1047    1784   +737