refactor unet_2d_condition tests

[akshan-main]

What does this PR do?

Part of the ongoing modeling-test migration (following #13369 and #13153). Migrates the UNet2DConditionModel test suite to the mixin-based structure (Config + ModelTesterMixin / TrainingTesterMixin / MemoryTesterMixin / AttentionTesterMixin / TorchCompileTesterMixin / LoraHotSwappingForModelTesterMixin). Last of the UNet migrations.

The model-specific tests (IP-adapter, custom diffusion, projection variants, sharded checkpoints, attn-procs warnings) are kept, the deprecated xformers and attention-slicing tests are dropped, and the @slow integration class is unchanged.

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 read our philosophy doc (important for complex PRs)?
• 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

[sayakpaul]

This one is here: #13153

[akshan-main]

Sounds good, I'll defer unet_2d_condition to #13153 since you've got a thorough version there already, closing this one.