Revision of BYOL module and tests

[matsumotosan]

What does this PR do?

Fixes part of #839

• pl_bolts.models.self_supervised.byol.byol_module.BYOL
• pl_bolts.models.self_supervised.byol.models.MLP
• pl_bolts.models.self_supervised.byol.models.SiameseArm
• pl_bolts.tests.models.self_supervised.test_models.test_byol

I cannot fix the CLI script as I do not have a GPU
pl_bolts.tests.models.self_supervised.test_models.test_ssl_scripts.test_cli_run_ssl_byol

Summary

• Add method in BYOL to calculate loss for given online and target views
• Add initial value of tau as an argument for BYOL
• Remove predictor network from SiameseArm since it was not being used for target network and move to BYOL
• Modify test_byol to use SimCLR transforms instead of CPC transforms
• Fix CLI run test case

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[otaj]

Hi, @matsumotosan! Thank you for this PR. I have to say, I love what you've done with byol_module in general, the meat of this module is now much more readable.

However, I left couple comments, which are mostly related to passing arguments around (similar to my comments in #873)

[matsumotosan]

@otaj I realized that the BYOL module is initialized with data_dir=datadir in test_byol. Should I re-open this PR to fix it or create a new PR?