Efficient gradient accumulation in LightningLite

[awaelchli]

What does this PR do?

Fixes #14713
Also sets the stage for #9234

# Accumulate gradient 8 batches at a time
is_accumulating = batch_idx % 8 != 0

with self.skip_backward_sync(model, enabled=is_accumulating):
    output = model(input)
    loss = ...
    self.backward(loss)
    ...

if not is_accumulating:
    optimizer.step()
    optimizer.zero_grad()

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

None.

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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda @carmocca @justusschock @awaelchli

[justusschock]

ltgm in general few comments regarding naming and errors

[codecov]

Codecov Report

Merging #14966 (d6a095a) into master (4acb10f) will increase coverage by 2%.
The diff coverage is 89%.

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #14966     +/-   ##
=========================================
+ Coverage      82%      84%     +2%     
=========================================
  Files         408      288    -120     
  Lines       29940    21959   -7981     
=========================================
- Hits        24686    18499   -6187     
+ Misses       5254     3460   -1794     

[tchaton]

LGTM !