Extract dataloader utilities from TrainerDataLoadingMixin

[awaelchli]

What does this PR do?

Addresses the suggestion by @ananthsub to move some static methods of the TrainerDataLoadingMixin to utilities:
#9987 (comment)

• Reduces the size of the TrainerDataLoadingMixin which we eventually want to remove anyway (Review Lightning architecture & API #7740)
• All moved methods were static and are now functions, so no side effects are produced.
• Code can be shared between Trainer and LightningLite without relying on importing TrainerDataLoadingMixin

Does your PR introduce any breaking changes? If yes, please list them.

No, because the methods were marked as "protected" (in reality they really are not, as we use them publicly. The "_" prefix is to discourage their use)

Before submitting

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• Did you make sure to update the documentation with your changes? (if necessary)
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PR review

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Did you have fun?

I made sure I had fun coding 🙃

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

cc @justusschock @awaelchli @akihironitta @tchaton @Borda

[carmocca]

Much better!

[tchaton]

LGTM !

[codecov]

Codecov Report

Merging #10145 (e729650) into master (c09c9c7) will increase coverage by 0%.
The diff coverage is 94%.

❗ Current head e729650 differs from pull request most recent head 626ff06. Consider uploading reports for the commit 626ff06 to get more accurate results

@@           Coverage Diff            @@
##           master   #10145    +/-   ##
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  Coverage      92%      92%            
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  Files         178      178            
  Lines       16286    16400   +114     
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+ Hits        14941    15057   +116     
+ Misses       1345     1343     -2