Fix support for plain LightningModules with FlashDataModule

[krshrimali]

What does this PR do?

Before this PR, there would be an error of missing attribute process_<train/val/test/predict>_dataset while using a Flash DataModule with plain LightningModule.

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• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests? [not needed for typos/docs]
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[krshrimali]

@ethanwharris - Regarding the doc update, can you help link the file/section that should be revised that it's now supported?

[codecov]

Codecov Report

Merging #1281 (2c3a35d) into master (2b2e4dd) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1281   +/-   ##
=======================================
  Coverage   91.79%   91.80%           
=======================================
  Files         285      285           
  Lines       12793    12793           
=======================================
+ Hits        11743    11744    +1     
+ Misses       1050     1049    -1     

Flag	Coverage Δ
unittests	91.80% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flash/core/data/data_module.py	94.52% <100.00%> (+0.36%)	⬆️

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[Borda]

lgtm