[bugfix] show_fix should set num_workers to 0

[tchaton]

What does this PR do?

Fixes # (issue)

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Codecov Report

❗ No coverage uploaded for pull request base (master@a756e8b). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #226   +/-   ##
=========================================
  Coverage          ?   86.61%           
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  Files             ?       57           
  Lines             ?     2750           
  Branches          ?        0           
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  Hits              ?     2382           
  Misses            ?      368           
  Partials          ?        0           

Flag	Coverage Δ
unittests	86.61% <100.00%> (?)

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Impacted Files	Coverage Δ
flash/vision/classification/data.py	87.80% <ø> (ø)
flash/data/data_module.py	78.09% <100.00%> (ø)

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[ethanwharris]

LGTM 😃 small nits

[edgarriba]

I'd add a small assert checking that internally the number of workers has been been kept as expected

[tchaton]

That's what _MultiProcessingDataLoaderIter does.