Refactor pl_bolts/datamodules

[akihironitta]

🚀 Refactoring

Currently, there are try: ... except: ... in pl_bolts/datamodules/__init__.py which were added in #303, and it is not clean in my humble opinion.

In order to make __init__.py clean and keep the behaviour, the following steps are needed:

1. Remove try: ... except: ... in __init__.py.
2. Rewrite optional imports (torchvision, sklearn, gym...) in pl_bolts/datamodules/*.py with importlib.util.find_spec("some_package") is not None in order not to raise ModuleNotFoundError with from pl_bolts.datamodules import ... The way of importing optional packages was suggested by @awaelchli in Raise ModuleNotFoundError when unavailable #290 (comment)

Additional context

These try: ... except: ... were added in #303 to fix the import problem in #291 where from pl_bolts.datamodules import MNISTDataModule raises ModuleNotFoundError even if the requirements for MNISTDataModule were met.

I would be happy to submit a PR if it sounds reasonable to you!

[ananyahjha93]

@akihironitta I think this would be a good enhancement. Go for it, send in the PR.

[Borda]

cc: @awaelchli