Smart handling of EarlyStopping.check_on_train_epoch_end

[carmocca]

What does this PR do?

When val_check_interval != 1.0, it's likely that early stopping should run after validation.

Same check as:

https://github.com/PyTorchLightning/pytorch-lightning/blob/5b143d02649c043ab7a7d0b444a42122f54f08d4/pytorch_lightning/callbacks/model_checkpoint.py#L251-L259

Could be considered a bugfix instead of a feature. Should it go into 1.4.x?

Reported in Slack: https://pytorch-lightning.slack.com/archives/CRBLFHY79/p1628241599256200

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

None

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[codecov]

Codecov Report

Merging #8888 (edbceb2) into master (5b143d0) will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #8888   +/-   ##
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  Coverage      93%     93%           
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  Files         169     169           
  Lines       14079   14082    +3     
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+ Hits        13053   13058    +5     
+ Misses       1026    1024    -2     

[awaelchli]

I believe not 1.4.x, because check_on_train_epoch_end was True so it seems this behavior was well defined.