Minor fixes to PyTorch Lightning

[arunppsg]

Description

This PR updated PyTorch Lightning to work with ModularTorchModel and it has to be merged in before #3745 .

A couple of other changes have also been made:

• added num workers arg to load dataset using multiple workers
• update pytorch-lightning to Lightning in imports as they are transitioning their namespace to Lightning (ref, example)
• Added documentation to DCLightningDataModule

Type of change

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
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• Documentations (modification for documents)

Checklist

• My code follows the style guidelines of this project
  • Run yapf -i <modified file> and check no errors (yapf version must be 0.32.0)
  • Run mypy -p deepchem and check no errors
  • Run flake8 <modified file> --count and check no errors
  • Run python -m doctest <modified file> and check no errors
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New unit tests pass locally with my changes
• I have checked my code and corrected any misspellings

[rbharath]

Let's add some tests for ModularTorchModel + lightning in a follow up PR

[rbharath]

LGTM