Default EarlyStopping.check_on_train_epoch_end=True

[carmocca]

What does this PR do?

After the changes in #7357 (not released yet), this is the safer default value as train_epoch_end will run after validation_end so it is more likely that the monitored value will have been logged. This assumes that most users want to run their ES callback at the end of the epoch and not somewhere in the middle.

Structure BEFORE #7357: running validation at the end of the epoch

on_train_epoch_end()
on_validation_end()
EarlyStopping.run_check()  # `check_on_train_epoch_end=False` by default

Structure AFTER #7357, BEFORE this PR: running validation at the end of the epoch

on_validation_end()
EarlyStopping.run_check()  # `check_on_train_epoch_end=False` by default
on_train_epoch_end()  # If your monitor was logged here, we broke your workflow

Structure AFTER #7357, AFTER this PR: running validation at the end of the epoch

on_validation_end()
on_train_epoch_end()
EarlyStopping.run_check()  # `check_on_train_epoch_end=True` by default

This matches the ModelPruning flag: https://github.com/PyTorchLightning/pytorch-lightning/blob/83039ba4705b02fbb55d5922d15a258a2f041986/pytorch_lightning/callbacks/pruning.py#L74
And one for ModelCheckpoint will be added in a follow-up PR.

Included some very minor typing/test changes

Part of #7724

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[codecov]

Codecov Report

Merging #8286 (83039ba) into master (7b6d0a8) will decrease coverage by 0%.
The diff coverage is 100%.

❗ Current head 83039ba differs from pull request most recent head 1d2170d. Consider uploading reports for the commit 1d2170d to get more accurate results

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- Coverage      93%     92%   -0%     
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- Hits        12752   12683   -69     
- Misses        977    1044   +67