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

_No response_

### System Info

Does not apply.

### Who can help?

_No response_