refactor unet_2d tests

[akshan-main]

What does this PR do?

Part of the ongoing modeling-test migration (following #13369 and #13153). Migrates the UNet2DModel test suites (the standard UNet, the LDM config, and NCSNpp) to the mixin-based structure (Config + ModelTesterMixin / TrainingTesterMixin / MemoryTesterMixin).

NCSNpp doesn't support layerwise casting, so those tests are skipped as before. The hub/pretrained integration tests are kept.

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?
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• Was this discussed/approved via a GitHub issue or the forum? (Discussed on Slack with @sayakpaul @DN6)
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• Did you write any new necessary tests?

Who can review?

@sayakpaul @DN6