Remove unused on_train_epoch_end hook in accelerator

[ananthsub]

What does this PR do?

We are auditing the Lightning components and APIs to assess opportunities for improvements:

• https://docs.google.com/document/d/1xHU7-iQSpp9KJTjI3As2EM0mfNHHr37WZYpDpwLkivA/edit#
• Review Lightning architecture & API #7740

No plugins or accelerators override this. Based on discussion in #9023: 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 directly removing this hook instead of going through a full deprecation cycle.

A more composable, (currently half-baked) approach could be:

• Split up ModelHooks into separate abstract classes that constitute pure interfaces.
• By default the LightningModule implements these hooks with no-ops. These remain overridable by end-users.
• Other components, like plugins, can also optionally implement these interfaces.
• We could somehow leverage this stronger interface contract/typing in Trainer.call_hook to actually run these hooks for components that opt-into using them them (could be stronger than the hasattr checks we have now @carmocca )

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

Yes. This removes on_train_epoch_end from the accelerator API. Custom accelerators not directly provided by the framework which were overriding this will no longer have this method called.

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[codecov]

Codecov Report

Merging #9035 (e983aa0) into master (13e64e6) will decrease coverage by 4%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #9035    +/-   ##
=======================================
- Coverage      93%     89%    -4%     
=======================================
  Files         175     175            
  Lines       14397   14396     -1     
=======================================
- Hits        13340   12755   -585     
- Misses       1057    1641   +584