Make the input `UNet2DConditionModel` and the returned `UNetMotionModel` instances share weights

**What API design would you like to have changed or added to the library? Why?**
[diffusers.UNetMotionModel](https://huggingface.co/docs/diffusers/main/en/api/models/unet-motion#diffusers.UNetMotionModel).from_unet2d

https://github.com/huggingface/diffusers/blob/6be43bd855772f5ffb064bc7c6049f578b3856f8/src/diffusers/models/unets/unet_motion_model.py#L431-L437

**New feature**
Add an option to make the input `UNet2DConditionModel` and the returned `UNetMotionModel` instances share weights (Res, SA, CA).

> Image from [PIA project page](https://pi-animator.github.io)

**What use case would this enable or better enable? Can you give us a code example?**
In the [example](https://huggingface.co/docs/diffusers/en/api/pipelines/pia#usage-example) in the current doc of PIA, users can provide an existing image to generate a video (img2vid). However, one can also perform text2vid by using an image pipeline to generate the input image first. In such case, it would be best if we only make **one copy** of the weights (Res, SA, CA) in both unets of image and video pipeline.

| Modules | Res | SA | CA | TA | `conv_in` (first 4 ch) | `conv_in` (last 5 ch) |
|-|-|-|-|-|-|-|
|`UNet2DConditionModel`|✅|✅|✅|❌|✅|❌|
|`UNetMotionModel`|✅|✅|✅|✅|✅|✅|

**Note**
The additional channels in `conv_in` takes neglectable amount of vram, and I don't know if it is possible to share only half of this module.

Start at src/diffusers/models/unets/unet_motion_model.py, especially the from_unet2d entry point around the linked lines, and inspect how the PIA usage example constructs the image and video pipelines. Define the option's behavior around sharing Res, SA, and CA weights while preserving the motion-specific modules and differing conv_in channels; done means the input UNet2DConditionModel and returned UNetMotionModel share those weights.

Escrito por el modelo de indexación a partir del texto del issue.