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)
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  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@sayakpaul @DN6