[Feat-BugFix] Resolve custom DataLoader

[tchaton]

What does this PR do?

This PR fixes the re-instantiation of custom DataLoader and provides a better error message

Fixes # (issue) <- this links related issue to this PR

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[pep8speaks]

Hello @tchaton! Thanks for updating this PR.

• In the file tests/trainer/test_data_loading.py:

Line 91:29: E203 whitespace before ':'

Comment last updated at 2021-02-04 20:52:40 UTC

[codecov]

Codecov Report

Merging #5745 (12084ad) into release/1.2-dev (d2c2e50) will decrease coverage by 0%.
The diff coverage is 97%.

@@               Coverage Diff                @@
##           release/1.2-dev   #5745    +/-   ##
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- Coverage               88%     88%    -0%     
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  Files                  181     181            
  Lines                12912   12775   -137     
================================================
- Hits                 11397   11248   -149     
- Misses                1515    1527    +12