Revision of MNIST module

[matsumotosan]

What does this PR do?

Fixes part of #839

• pl_bolts.models.mnist_module.LitMNIST
• pl_bolts.models.mnist_module.cli_main
• pl_bolts.tests.models.test_mnist_templates.test_mnist
• pl_bolts.tests.callbacks.test_data_monitor.test_base_log_interval_override
• pl_bolts.tests.callbacks.test_data_monitor.test_base_log_interval_fallback

Summary

• Remove dataloader related methods from LitMNIST and use MNISTDataModule instead
• Add shared step to LitMNIST
• Add docstring to LitMNIST
• Fix test_mnist
• Fix test_data_monitor tests by initializing MNISTDataModule inside test case since dataloaders were removed from LitMNIST

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• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests? [not needed for typos/docs]
• Did you verify new and existing tests pass locally with your changes?
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[otaj]

Hi, @matsumotosan, thank you for the PR! It looks good, I have just one small nit

[matsumotosan]

Hi @otaj, I have pulled in the changes from #890 and it seems to be working, but the tests are failing for macOS for test_mnist and I am not quite sure how to fix it. Do you mind taking a look?

[otaj]

Hi, we need to silence one warning on mac, because the machine has multiple CPU cores. I thought I fixed it in 9960a75, but it seems, I forgot the format for silencing a warning is trying to match the beginning of the message. It should be fixed by now, the tests pass on my machine with multiple CPU cores, we'll see if the tests pass on the CI too :)