Fix mypy typing for utilities.debugging

[stancld]

What does this PR do?

Part of #7037. This PR disentangles large #7218. It fixes mypy typing in utilities.device_parser.py.

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

None

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[codecov]

Codecov Report

Merging #8672 (d1923c4) into master (9ca02f5) will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #8672   +/-   ##
======================================
  Coverage      89%     89%           
======================================
  Files         168     168           
  Lines       14044   14048    +4     
======================================
+ Hits        12461   12465    +4     
  Misses       1583    1583           

[carmocca]

There's one mypy error

pytorch_lightning/utilities/debugging.py:173: error: Item "None" of "Optional[ModelCheckpoint]" has no attribute "monitor" [union-attr]

[stancld]

@carmocca

There's one mypy error

pytorch_lightning/utilities/debugging.py:173: error: Item "None" of "Optional[ModelCheckpoint]" has no attribute "monitor" [union-attr]

The return type of trainer.checkpoint_callback is defined as Optional[ModelCheckpoint] from

/pytorch_lightning/trainer/properties.py:383:    def checkpoint_callback(self) -> Optional[ModelCheckpoint]:

Can we here always assume checkpoint_callback is not None?

[carmocca]

Can we here always assume checkpoint_callback is not None?

No, users can disable it by setting Trainer(checkpoint_callback=False)

[tchaton]

LGTM !