Fix retrieval of batch indices when dataloader num_workers > 0

[awaelchli]

What does this PR do?

Fixes #10782

Please see my comment here #10782 (comment) for a detailed answer where the bug comes from and why we need to change the way we use the IndexBatchSamplerWrapper. In short, the way a PyTorch BatchSampler retrieves the batch indices differs between num_workers=0 and num_workers > 0 in the DataLoader, and our prediction loop needs to account for that.

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

No.

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Did you have fun?

I made sure I had fun coding 🙃

Part of #1 (it's a lie, this is just here to avoid noisy GitHub bot)

cc @Borda

[tchaton]

LGTM !

[codecov]

Codecov Report

Merging #10870 (ed735c0) into master (541b983) will increase coverage by 0%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #10870   +/-   ##
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  Coverage      92%      92%           
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  Files         177      177           
  Lines       16567    16579   +12     
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+ Hits        15206    15218   +12     
  Misses       1361     1361