Commented train_datalaoder in data_connector, and reset_dataloader ex…

[gianscarpe]

…plicitely set train_dataloader from datamodule (if available). confg_validation checks if datamodule is available and eventually check datamodule.train_dataloader is overridden

What does this PR do?

Fixes #<issue_number>

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[codecov]

Codecov Report

Merging #6103 (de60a3b) into master (0b27147) will decrease coverage by 60%.
The diff coverage is 8%.

@@           Coverage Diff            @@
##           master   #6103     +/-   ##
========================================
- Coverage      93%     33%    -60%     
========================================
  Files         160     160             
  Lines       11402   11304     -98     
========================================
- Hits        10654    3732   -6922     
- Misses        748    7572   +6824     

[pep8speaks]

Hello @gianscarpe! Thanks for updating this PR.

• In the file pytorch_lightning/trainer/data_loading.py:

Line 209:121: E501 line too long (140 > 120 characters)

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