Loop specialization

[awaelchli]

What does this PR do?

Enables loop specialization through a connect mechanism.
Three main use cases:

A) Set loop directly on the trainer:

class MyFitLoop(FitLoop):
    # this loop has no nested loops
    ...

trainer = Trainer()
trainer.fit_loop = MyFitLoop()

B) Connect child loops to the main loop (nesting):

class MyFitLoop(FitLoop):
    # this loop defines subloops
    
    def connect(self, child0, child1):
        self.child0 = child0
        ...

trainer = Trainer()
fit_loop = MyFitLoop()
child_loop = MyChildLoop()
fit_loop.connect(child_loop)

trainer.fit_loop = fit_loop

C) Connect a custom loop to an existing Lightning loop:

class CustomBatchLoop(TrainingBatchLoop):
    # this loop is a custom implementation of the Lightning built-in batch loop
    ...

trainer = Trainer()
my_batch_loop = CustomBatchLoop()
trainer.fit_loop.epoch_loop.connect(my_batch_loop)

Setting the trainer references is handled internally by Lightning. The user does NOT connect the trainer directly to the loop.

Before submitting

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• Did you make sure your PR does only one thing, instead of bundling different changes together?
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• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
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Did you have fun?

I made sure I had fun coding 🙃

[pep8speaks]

Hello @awaelchli! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-07-19 11:55:06 UTC

[codecov]

Codecov Report

Merging #8226 (8a58e01) into master (7bb810f) will increase coverage by 0%.
The diff coverage is 97%.

@@           Coverage Diff           @@
##           master   #8226    +/-   ##
=======================================
  Coverage      92%     92%            
=======================================
  Files         217     217            
  Lines       14094   14341   +247     
=======================================
+ Hits        13001   13236   +235     
- Misses       1093    1105    +12     

[tchaton]

Hey @ananthsub @awaelchli, what are your thoughts there ?

[tchaton]

Looks neat !