Add CacheMixin to Wan and LTX Transformers

[DN6]

What does this PR do?

Fixes # (issue)
#11134

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[a-r-r-o-w]

Thanks!

[a-r-r-o-w]

Just want to note that cache methods may not work with Wan as expected. The implementations are very brittle at the moment, but I'm working on improving this with #11180 and follow up PRs

Also could probably enable cache tests for Wan as well.

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