fix mypy typing errors in pytorch_lightning/tuner/lr_finder.py

[donlapark]

What does this PR do?

Fixes mypy typing errors in pytorch_lightning/tuner/lr_finder.py in #13445.

• Import of matplotlib has been moved to the beginning of the file in order to enable plt.Figure type.
• Line 129: The type of scheduler in LRSchedulerConfig is expected to be Union[_LRScheduler, ReduceLROnPlateau], but both _LinearLR and _ExponentialLR inherit from torch.optim.lr_scheduler._LRScheduler.
• Line 209: sometimes mypy will throw Return value expected error when it detects a lone return (Return value expected on def f() -> Optional[Type]: with plain return python/mypy#1003), in which case I added # type: ignore to the line.
• Line 231: mypy will throw in an error if a function is assigned to a method (Unable to assign a function a method python/mypy#2427).
• Line 332: Union[_LRScheduler, ReduceLROnPlateau] has no attribute lr.

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

None

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[akihironitta]

Thank you for working on this!

[donlapark]

Sorry for the closing and reopening PR -- most of the checks are stuck with Expected — Waiting for status to be reported so I am trying to unstuck it.

[justusschock]

Sorry for the closing and reopening PR -- most of the checks are stuck with Expected — Waiting for status to be reported so I am trying to unstuck it.

That is because as a first-time contributor you need approval from our team to run our CI (which I just approved)