Add LRFinder callback

[rohitgr7]

What does this PR do?

Part of #11012
will add docs and examples in a follow up.

Added LearningRateFinder callback that allows users to run lr_finder at any time they want during .fit call using a callback.

class CustomLearningRateFinder(LearningRateFinder):
    def on_fit_start(self, trainer, pl_module):
        self.lr_find(trainer, pl_module)
    
    def on_train_epoch_start(self, trainer, pl_module):
        if self.trainer.current_epoch == 10:
            self.lr_find(trainer, pl_module)


lr_cb = CustomLearningRateFinder(...)
trainer = Trainer(..., callbacks=lr_cb)
trainer.fit(...)

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

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cc @Borda @justusschock @awaelchli @rohitgr7 @akihironitta

[awaelchli]

Great work @rohitgr7
Team (@Borda @carmocca @tchaton @kaushikb11 etc. can we bring these two PRs in? They have been waiting for a long time. I know we are all not well familiar with the tuner, so let's just review as best as we can and let @rohitgr7 finalize the work?

[Borda]

Great work @rohitgr7 Team (@Borda @carmocca @tchaton @kaushikb11 etc. can we bring these two PRs in? They have been waiting for a long time. I know we are all not well familiar with the tuner, so let's just review as best as we can and let @rohitgr7 finalize the work?

That would be GREAT!

[Borda]

Can we add docs on how to use this cb? 🦦

[Borda]

@rohitgr7, mind checking the collisions? 🦦