Fix data fetcher selection

[awaelchli]

What does this PR do?

Fixes #11266

What does this PR do?

Fixes

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

cc @Borda @justusschock @awaelchli @ninginthecloud

[rohitgr7]

is it possible to add a test for the fix?

[awaelchli]

The data fetcher code and inter-batch parallelism is untested as far as I know. The Trainer and connectors are currently not well unit-testable, I predict this code will not survive very long and I would avoid adding tests for checking internal attributes that get set this way.

[codecov]

Codecov Report

Merging #11294 (a4a6090) into master (1fc046c) will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #11294   +/-   ##
=======================================
  Coverage      92%      92%           
=======================================
  Files         177      177           
  Lines       16840    16840           
=======================================
  Hits        15479    15479           
  Misses       1361     1361           

[Borda]

@awaelchli shall we cover it by a test?