Add ModelCheckpoint(save_on_train_epoch_end)

[carmocca]

What does this PR do?

Adds ModelCheckpoint(save_on_train_epoch_end) which works just as EarlyStopping(check_on_train_epoch_end) and ModelPruning(prune_on_train_epoch_end)

The flag defaults to True if the Trainer(val_check_interval) flag is unchanged. This is because if you are running validation multiple times per training epoch, you probably want to save right after. Otherwise, saving on_train_epoch_end is the safer option as it is the last hook called so it's more likely the monitor will have been logged.

Part of #7724
Closes #6671

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

If your workflow required running the ModelCheckpoint save exactly after validation ends, this will break it unless save_on_train_epoch_end=False is specified.

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[pep8speaks]

Hello @carmocca! Thanks for updating this PR.

• In the file pytorch_lightning/callbacks/model_checkpoint.py:

Line 306:13: W503 line break before binary operator

Comment last updated at 2021-07-13 14:16:08 UTC

[codecov]

Codecov Report

Merging #8389 (101368d) into master (000fbe6) will decrease coverage by 1%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #8389   +/-   ##
======================================
- Coverage      93%     92%   -1%     
======================================
  Files         216     216           
  Lines       14115   14123    +8     
======================================
- Hits        13087   13023   -64     
- Misses       1028    1100   +72     

[tchaton]

LGMT !