Remove TrainingTypePlugin.on_save and Accelerator.on_save

[four4fish]

What does this PR do?

Fixes #8987
Double checked none of the plugins overridden on_save()

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

Removes on_save from TrainingTypePlugin and Accelerator

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[ananthsub]

thanks for picking this up @four4fish !

@PyTorchLightning/core-contributors I'm not sure what the deprecation policies & versioning guarantees are for plugins & accelerators.

the docs say this:

The Plugin API is in beta and subject to change. For help setting up custom plugins/accelerators, please reach out to us at support@pytorchlightning.ai

https://pytorch-lightning.readthedocs.io/en/latest/extensions/plugins.html

Does this mean we can make the change in-place like this?

deprecation policy aside, this PR is what we'll end up doing.

[codecov]

Codecov Report

Merging #9023 (49825c7) into master (13e64e6) will decrease coverage by 4%.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master   #9023    +/-   ##
=======================================
- Coverage      93%     89%    -4%     
=======================================
  Files         175     175            
  Lines       14397   14391     -6     
=======================================
- Hits        13340   12750   -590     
- Misses       1057    1641   +584     

[ananthsub]

Discussing with @carmocca - the plugins & accelerator API is not stable enough yet for it to warrant full backward compatibility. the blast radius of this change is small given none of the framework's implementations are affected. Therefore, I'm in favor of directly removing this.