Fix save/load/resume from checkpoint for DeepSpeed Plugin

[SeanNaren]

What does this PR do?

Closes #7282 #8344 #8092

Re-write of #7297

This PR will fix DeepSpeed saving/loading and resuming.

TLDR: DeepSpeed ZeRO Stage 3 was broken when saving/loading. This is now fixed. If the user wants to resume from checkpoint they need to set DeepSpeedPlugin(save_full_weights=False) when using ZeRO Stage 3. Documentation will be updated once PR is merged!

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)
• Did you list all the breaking changes introduced by this pull request?

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 🙃

[kaushikb11]

Disabled auto-merge if you plan to push few more changes. LGTM!