refactor autoencoder tests (asymmetric_kl, ltx_video)

[akshan-main]

What does this PR do?

Part of the ongoing modeling-test migration (following #13369 and #13153). Migrates the AsymmetricAutoencoderKL and AutoencoderKLLTXVideo test suites to the mixin-based structure (Config + ModelTesterMixin / TrainingTesterMixin / MemoryTesterMixin / NewAutoencoderTesterMixin).

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Who can review?

@sayakpaul @DN6

[DN6]

Thanks @akshan-main