UNet3DConditionModel does not use class conditioning

### Describe the bug

Hello guys,

I found a small inconsistency. In `UNet3DConditionModel` forward method, `class_labels` is [defined](https://github.com/huggingface/diffusers/blob/6fedbd850a282ef21009f99a394f619ed7420137/src/diffusers/models/unets/unet_3d_condition.py#L540C13-L540C25) but is never used after that.

This is different from its twin model `UNet2DConditionModel`, where it is [used](https://github.com/huggingface/diffusers/blob/6fedbd850a282ef21009f99a394f619ed7420137/src/diffusers/models/unets/unet_2d_condition.py#L992) to get an embedding which is later used by the model.

Cheers.

### Reproduction

Does not apply.

### Logs

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### System Info

Does not apply.

### Who can help?

_No response_

Start in src/diffusers/models/unets/unet_3d_condition.py at the UNet3DConditionModel forward method and compare its class_labels handling with the linked UNet2DConditionModel implementation. Trace how the class embedding is obtained and consumed in the 2D model. Done means class conditioning is handled consistently in the 3D model and the relevant behavior is covered by the project's existing model tests.

Scritto dal modello di indicizzazione a partire dal testo della issue.