BYOL weight update callback

[matsumotosan]

What does this PR do?

Fixes part of #839

• pl_bolts.callbacks.byol_updates.BYOLMAWeightUpdate

Summary

• Modify update_tau method to update tau value inside method instead of returning new tau value
• Add more tests for initial values of tau
  • Invalid values (<0, >1) -> raises ValueError
  • Edge cases (0, 1) -> equal to online/target, respectively
  • In between 0 and 1 -> test for exact value of parameters

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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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Did you have fun?

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[otaj]

It looks good, only one major comment regarding catch_warnings. Once that's solved (and no warnings are raised, I'll be happy to give final approval :)

Thank you! ⚡