Updated iterable dataset with len warning message

[ozen]

Based on pytorch docs about dataloader length and iterable datasets.

What does this PR do?

Fixes #6966

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[pep8speaks]

Hello @ozen! Thanks for updating this PR.

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

Comment last updated at 2021-04-13 00:05:21 UTC

[codecov]

Codecov Report

Merging #6972 (1a82a09) into master (030b76b) will decrease coverage by 5%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #6972    +/-   ##
=======================================
- Coverage      92%     86%    -5%     
=======================================
  Files         194     194            
  Lines       12356   12385    +29     
=======================================
- Hits        11337   10706   -631     
- Misses       1019    1679   +660