Ref: Pull duplicate data interface definition up into DataHooks class

[awaelchli]

What does this PR do?

Fixes #2707

The PyTorch Lightning codebase must maintain a high quality. Code duplication is a source of bugs, hard to maintain and challenges the sanity of all developers involved.

By pulling the data related hooks up into a common super class / interface, we can maintain the documentation and specification in one place while implementing specific behaviour in subclasses. For example, the datamodule defines the dataloader methods as abstract, while the LightningModule does not. We can override the methods in the DataModule base class and decorate them as abstract, while still inheriting the docstring from the super class.

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[codecov]

Codecov Report

Merging #3344 into master will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #3344   +/-   ##
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  Coverage      90%     90%           
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  Files          91      91           
  Lines        8157    8155    -2     
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  Hits         7366    7366           
+ Misses        791     789    -2