[bugfix] Add support for CombinedLoader in validation with ddp

[tchaton]

What does this PR do?

This PR add support resolve a bug with CombinedLoader and DDP.

Fixes #7013

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[pep8speaks]

Hello @tchaton! Thanks for updating this PR.

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

Comment last updated at 2021-04-20 07:52:39 UTC

[codecov]

Codecov Report

Merging #7102 (a83fee2) into master (d12c6cf) will decrease coverage by 5%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #7102    +/-   ##
=======================================
- Coverage      92%     87%    -5%     
=======================================
  Files         197     197            
  Lines       12634   12686    +52     
=======================================
- Hits        11649   11039   -610     
- Misses        985    1647   +662     

[awaelchli]

I don't understand why we are testing the DistributedSampler in a full special test.
Would a simpler test do it? We have similar tests in tests/trainer/test_dataloaders

dataloader = CombinedLoader()
trainer = Trainer()
dataloader = trainer.auto_add_sampler(dataloader)
assert all(isinstance(loader.sampler, DistributedSampler) for loader in dataloader.loaders))

[tchaton]

Great comment ! You are definitely right ! Let me update the test.