refactor autoencoder tests (vq, kvae_video, oobleck, consistency_decoder, tiny, vidtok)

[akshan-main]

What does this PR do?

Part of the ongoing modeling-test migration (following #13369 and #13153). Migrates the VQModel, AutoencoderKLKVAEVideo, AutoencoderOobleck, ConsistencyDecoderVAE, AutoencoderTiny, and AutoencoderVidTok test suites to the mixin-based structure (Config + ModelTesterMixin / TrainingTesterMixin / MemoryTesterMixin / NewAutoencoderTesterMixin).

@slow integration classes in the oobleck, consistency_decoder, and tiny suites are kept intact.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline?
• Did you read our philosophy doc (important for complex PRs)?
• Was this discussed/approved via a GitHub issue or the forum? (Discussed on Slack with @DN6)
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@sayakpaul @DN6