Delete TrainingEpochLoop._dataloader_idx which always equals 0

[carmocca]

What does this PR do?

See title.

A follow-up could deprecate the argument for on_train_batch_{start,end}.

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

None

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?
• [n/a] Did you make sure to update the documentation with your changes? (if necessary)
• [n/a] 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 list all the breaking changes introduced by this pull request?
• [n/a] Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

• 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

[codecov]

Codecov Report

Merging #8911 (e3c3cc5) into master (0aa5cc7) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #8911   +/-   ##
======================================
- Coverage      93%     93%   -0%     
======================================
  Files         172     172           
  Lines       14114   14111    -3     
======================================
- Hits        13094   13091    -3     
  Misses       1020    1020