Binary MNIST/EMNIST Datasets and Datamodules

[matsumotosan]

What does this PR do?

Fixes part of #839

• pl_bolts.datamodules.binary_emnist_datamodule.BinaryEMNISTDataModule
• pl_bolts.datamodules.binary_mnist_datamodule.BinaryMNISTDataModule
• pl_bolts.datasets.emnist_dataset.BinaryEMNIST (add unit test)
• pl_bolts.datasets.mnist_dataset.BinaryMNIST (add unit test)

Summary

• Add tests for binarized MNIST and EMNIST datasets
• Add threshold attribute (set to 127) to Dataset classes to be used for binarizing MNIST and EMNIST images
• Add tests for val and test dataloaders

By default, the images are returned as PIL images (mode='L') in line with the torchvision MNIST class. The threshold value is manually set as of right now.

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[otaj]

Over all, it looks very good, I have just two nits left in the comments