[PRX] Improve model compilation

[WaterKnight1998]

What does this PR do?

This PR reimplements the img2seq and seq2img utilities in the PRX model to enable successful ONNX export and TensorRT inference.
Previously, these utilities depended on torch.nn.functional.fold and torch.nn.functional.unfold. During ONNX export, this resulted in the use of the Col2Im ONNX operator, which is not supported by the TensorRT ONNX operator set, causing engine building to fail.

The new implementation removes the dependency on Col2Im while preserving the expected functionality and output shapes, ensuring compatibility across ONNX runtimes and TensorRT. The approach follows the unpatchify logic used elsewhere in the library, for example in dit_transformer_2d.py.

Fixes #12786

Code to help you verify

Export model to onnx

import torch
from diffusers.models.transformers.transformer_prx import PRXTransformer2DModel


# Load model
model = PRXTransformer2DModel.from_pretrained("Photoroom/prx-1024-t2i-beta", subfolder="transformer", torch_dtype=torch.bfloat16).to("cuda")

batch_size = 2 # for guidance
device = torch.device("cuda")

# Prepare input

dummy_inputs = {
    "hidden_states": torch.randn(batch_size, model.config.in_channels, 128, 128, dtype=torch.bfloat16, device=device), # Input latent image tensor of shape `(B, C, H, W)`.
    "timestep": torch.tensor([1.0] * batch_size, dtype=torch.float32, device=device), #  Timestep tensor of shape `(B,)` or `(1,)`, used for temporal conditioning.
    "encoder_hidden_states": torch.randn(batch_size, 256, model.config.context_in_dim, dtype=torch.bfloat16, device=device), # Text conditioning tensor of shape `(B, L_txt, context_in_dim)`.
    "attention_mask": torch.randint(0, 2, (batch_size, 256), dtype=torch.int64, device=device), #  Boolean mask of shape `(B, L_txt)`, where `0` marks padding in the text sequence.
}

output_names = ["latent"]

batch_dim = torch.export.Dim("batch_size", min=1, max=16) 

dynamic_shapes = {
    "hidden_states": {0: batch_dim},
    "timestep": {0: batch_dim},
    "encoder_hidden_states": {0: batch_dim},
    "attention_mask": {0: batch_dim} 
}

# Export it
torch.onnx.export(
    model,
    tuple(dummy_inputs.values()), 
    "prx.onnx",
    input_names=list(dummy_inputs.keys()),
    output_names=output_names,
    dynamic_shapes=dynamic_shapes,
    opset_version=20,
    dynamo=True,
    do_constant_folding=False,
    verbose=True
)

Convert model to tensorrt

docker run --rm --gpus all -it \
    -v "$(pwd):/workspace" \
    -w /workspace \
    nvcr.io/nvidia/tensorrt:25.11-py3 \
    trtexec \
    --onnx=prx.onnx \
    --saveEngine=prx_engine.plan \
    --fp16 \
    --memPoolSize=workspace:16384 \
    --minShapes=hidden_states:2x16x128x128,timestep:2,encoder_hidden_states:2x256x2304,attention_mask:2x256 \
    --optShapes=hidden_states:2x16x128x128,timestep:2,encoder_hidden_states:2x256x2304,attention_mask:2x256 \
    --maxShapes=hidden_states:2x16x128x128,timestep:2,encoder_hidden_states:2x256x2304,attention_mask:2x256 \
    --verbose

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@sayakpaul @DavidBert @qgallouedec

[sayakpaul]

@DavidBert could you review this?

@WaterKnight1998 I am not sure why you tagged Quentin on this PR.

[sayakpaul]

Changes seem safe for me to merge!

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[sayakpaul]

@bot /style

[github-actions]

Style bot fixed some files and pushed the changes.

[DavidBert]

LGTM! Thanks

[WaterKnight1998]

@sayakpaul PR is ready for merge