fix case where an IterableDataset doesn't produce a batch for an epoch

[awaelchli]

What does this PR do?

Fixes #6810

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[pep8speaks]

Hello @awaelchli! Thanks for updating this PR.

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

Comment last updated at 2021-04-30 11:30:33 UTC

[codecov]

Codecov Report

Merging #7294 (396da1a) into master (338f5a3) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #7294   +/-   ##
======================================
- Coverage      91%     91%   -0%     
======================================
  Files         199     199           
  Lines       12797   12806    +9     
======================================
- Hits        11678   11671    -7     
- Misses       1119    1135   +16     

[ethanwharris]

LGTM 😃

[tchaton]

LGTM !