ZImageControlNet.from_transformer creates a shallow copy of the transformer weights

### Describe the bug

When using the [.from_transformer](https://github.com/huggingface/diffusers/blob/1b8fc6c5895ec73f29a5a3caf05c4fdc60450085/src/diffusers/models/controlnets/controlnet_z_image.py#L519) classmethod of the [ZImageControlNetModel](https://github.com/huggingface/diffusers/blob/1b8fc6c5895ec73f29a5a3caf05c4fdc60450085/src/diffusers/models/controlnets/controlnet_z_image.py#L432), the layers that are copied from the transformer are shallow copies. This means that any changes done to the controlnet weights after copying will also affect the transformer weights.

I'm unclear on what the best solution would be. Looking at [FluxControlNetModen.from_transformer](https://github.com/huggingface/diffusers/blob/430c557b6a66a3c2b5740fb186324cb8a9f0f2e9/src/diffusers/models/controlnets/controlnet_flux.py#L122), they have already prepared the layers in the `__init__` function, and therefore can directly load in using state dicts, but this is harder here, given how the `__init__` and `from_transformer` functions have been made in `ZImageControlNetModel`. Short term, I've been able to circumvent it by using `copy.deepcopy(...)`, but I think something more elegant is possible here.

### Reproduction

```python
import torch
from diffusers import ZImageControlNetModel, ZImageTransformer2DModel

transformer = ZImageTransformer2DModel.from_pretrained(
"Tongyi-MAI/Z-Image",
subfolder="transformer",
torch_dtype=torch.bfloat16,
)

controlnet = ZImageControlNetModel(
control_layers_places=[0, 15, 29], # These parameters are not relevant, just for instantiation
control_refiner_layers_places=[0, 1], # ^
add_control_noise_refiner="control_noise_refiner", # ^
control_in_dim=16 # ^
)

controlnet = ZImageControlNetModel.from_transformer(
controlnet=controlnet,
transformer=transformer
)

# Weights before modification
print("Transformer t_embedder weights before modification:")
print(transformer.t_embedder.mlp[0].weight)

# Fill ControlNet t_embedder weights with constant value
torch.nn.init.constant_(controlnet.t_embedder.mlp[0].weight, 42.0)
print("ControlNet t_embedder weights after modification:")
print(controlnet.t_embedder.mlp[0].weight) # As expected, filled with 42s

# Should remain unchanged, but is also filled with 42s
print("Transformer t_embedder weights after ControlNet modification:")
print(transformer.t_embedder.mlp[0].weight)
```

### Logs

```shell
Transformer t_embedder weights before modification:
Parameter containing:
tensor([[ 0.0069, -0.0051, 0.0036, ..., 0.0325, -0.0153, -0.0089],
[ 0.0004, -0.0102, 0.0078, ..., 0.0262, 0.0229, 0.0505],
[-0.0024, 0.0032, -0.0040, ..., 0.0237, 0.0195, 0.0017],
...,
[-0.0059, -0.0013, 0.0008, ..., 0.0223, 0.0209, 0.0132],
[ 0.0026, -0.0014, -0.0052, ..., 0.0021, 0.0142, 0.0630],
[-0.0004, 0.0034, 0.0032, ..., 0.0339, 0.0187, 0.0254]],
dtype=torch.bfloat16, requires_grad=True)
ControlNet t_embedder weights after modification:
Parameter containing:
tensor([[42., 42., 42., ..., 42., 42., 42.],
[42., 42., 42., ..., 42., 42., 42.],
[42., 42., 42., ..., 42., 42., 42.],
...,
[42., 42., 42., ..., 42., 42., 42.],
[42., 42., 42., ..., 42., 42., 42.],
[42., 42., 42., ..., 42., 42., 42.]], dtype=torch.bfloat16,
requires_grad=True)
Transformer t_embedder weights after ControlNet modification:
Parameter containing:
tensor([[42., 42., 42., ..., 42., 42., 42.],
[42., 42., 42., ..., 42., 42., 42.],
[42., 42., 42., ..., 42., 42., 42.],
...,
[42., 42., 42., ..., 42., 42., 42.],
[42., 42., 42., ..., 42., 42., 42.],
[42., 42., 42., ..., 42., 42., 42.]], dtype=torch.bfloat16,
requires_grad=True)
```

### System Info

- 🤗 Diffusers version: 0.37.0.dev0
- Platform: Linux-5.15.0-91-generic-x86_64-with-glibc2.35
- Running on Google Colab?: No
- Python version: 3.11.14
- PyTorch version (GPU?): 2.9.0+cu126 (True)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed
- Huggingface_hub version: 0.35.3
- Transformers version: 4.57.1
- Accelerate version: 1.11.0
- PEFT version: 0.17.1
- Bitsandbytes version: 0.48.2
- Safetensors version: 0.6.2
- xFormers version: not installed
- Accelerator: NVIDIA RTX 6000 Ada Generation, 49140 MiB
NVIDIA RTX A6000, 49140 MiB
NVIDIA RTX A6000, 49140 MiB
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

### Who can help?

_No response_

Start in src/diffusers/models/controlnets/controlnet_z_image.py at ZImageControlNetModel.from_transformer and its __init__, then compare the construction used by FluxControlNetModel.from_transformer in src/diffusers/models/controlnets/controlnet_flux.py. Run the supplied reproduction to identify which parameters share storage. Done means modifying copied ControlNet weights no longer changes the original transformer weights.

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