pytorch_lightning.loops file structure: group by dataloader, epoch, and batch loop

[awaelchli]

What does this PR do?

part of #7938

The current structure is the following:

loops/
    dataloader/
        dataloader_loop.py
        evaluation_dataloader_loop.py
        prediction_dataloader_loop.py
    base.py
    fit_loop.py
    training_epoch_loop.py
    evaluation_epoch_loop.py
    prediction_epoch_loop.py
    training_batch_loop.py

The new proposed file structure:

loops/
    dataloader/
        dataloader_loop.py
        evaluation_loop.py
        prediction_loop.py
    epoch/
        training_epoch_loop.py
        evaluation_epoch_loop.py
        prediction_epoch_loop.py

    batch/
        training_batch_loop.py
        
    base.py
    fit_loop.py

No deprecations required. The new loops were introduced in master only so far and are for internal use currently.

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[codecov]

Codecov Report

Merging #8077 (de1297a) into master (8bd7b1b) will decrease coverage by 4%.
The diff coverage is 95%.

@@           Coverage Diff           @@
##           master   #8077    +/-   ##
=======================================
- Coverage      92%     88%    -4%     
=======================================
  Files         211     211            
  Lines       13658   13416   -242     
=======================================
- Hits        12512   11803   -709     
- Misses       1146    1613   +467     

[tchaton]

LGTM !

[carmocca]

Still need to move this inside the epoch loop as we discussed.

Or do you want to do that in a separate PR?

[awaelchli]

yes, I have it already implemented locally,
there is going to be a circular import due to that, so I would like to discuss the resolution of that in a separate PR.

Changes were discussed