Revision of SimSiam module and tests

[matsumotosan]

What does this PR do?

Fixes part of #839

Summary

• Remove parts unrelated to model from SimSiam
• Set default model parameters to those from the original paper
• Test revised module

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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?
• N/A If you made a notable change (that affects users), did you update the CHANGELOG?

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[otaj]

I feel like I've already reviewed this 😄 It looks good, to be fair, thank you 🔩