Resolve FitLoop.done TODO

[carmocca]

What does this PR do?

Moves min step checking logic into the loop that manages steps

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

None intended

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cc @justusschock @awaelchli @akihironitta @rohitgr7 @carmocca @ananthsub @ninginthecloud

[awaelchli]

Is this covered by test_min_steps_override_early_stopping_functionality?

Assuming trainer.should_stop=True, does the following hold true?

Trainer(limit_train_batches=5, min_steps=3, min_epochs=2)   # calls training_step 10 times
Trainer(limit_train_batches=5, min_steps=12, min_epochs=2)   # calls training_step 12 times

[carmocca]

Is this covered by test_min_steps_override_early_stopping_functionality?

No, but it is after #11866

does the following hold true?

The second one will not.

If that requirement cannot change, we have two options:

1. Revisit this after [WIP] Add Loop.stop() #8604 so that a parent loop would check the children's should_stop property.
2. Just remove this TODO as it cannot be done given the current design.

In any case, I'll mark this as a draft. Tell me what do you think