FashionMNIST/EMNIST Datamodules

[matsumotosan]

What does this PR do?

Fixes part of #839

• pl_bolts.datamodules.emnist_datamodule.EMNISTDataModule
• pl_bolts.datamodules.fashion_mnist_datamodule.FashionMNISTDataModule

Summary

• Remove under review warning tags for datamodules
• Clean docstrings

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• Did you write any new necessary tests? [not needed for typos/docs]
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PR review

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[otaj]

I left one very minor comment, otherwise it looks good!