`Cosmos2VideoToWorldPipeline` applies FPS RoPE modulation that the released Cosmos-Predict2 checkpoints disable natively (default fps=16 → temporal RoPE ×1.5)

### Describe the bug

`CosmosRotaryPosEmbed` scales its temporal frequencies by FPS whenever `fps is not None`:

https://github.com/huggingface/diffusers/blob/main/src/diffusers/models/transformers/transformer_cosmos.py#L505-L512

```python
if fps is None:
emb_t = torch.outer(seq[: pe_size[0]], temporal_freqs)
else:
emb_t = torch.outer(seq[: pe_size[0]] / fps * self.base_fps, temporal_freqs) # base_fps = 24
```

and `Cosmos2VideoToWorldPipeline` always feeds it — `fps: int = 16` by default:

https://github.com/huggingface/diffusers/blob/main/src/diffusers/pipelines/cosmos/pipeline_cosmos2_video2world.py#L498
https://github.com/huggingface/diffusers/blob/main/src/diffusers/pipelines/cosmos/pipeline_cosmos2_video2world.py#L713

However, in the reference implementation this modulation sits behind an `enable_fps_modulation`
flag, and the **released Cosmos-Predict2 checkpoints ship with it turned off**:

- rope implementation (guarded): https://github.com/nvidia-cosmos/cosmos-predict2/blob/main/cosmos_predict2/models/text2image_dit.py#L541-L552
- 2B Video2World net config: https://github.com/nvidia-cosmos/cosmos-predict2/blob/main/cosmos_predict2/configs/base/config_video2world.py#L117 (`rope_enable_fps_modulation=False`)
- 14B Video2World net config: same file, L207
- Text2Image nets: https://github.com/nvidia-cosmos/cosmos-predict2/blob/main/cosmos_predict2/configs/base/config_text2image.py#L110 (also L231, L309)

So natively these models use raw integer temporal RoPE positions and ignore FPS entirely,
while the diffusers port has no such flag and, with the pipeline default `fps=16` against
`base_fps=24`, runs every denoising step with temporal RoPE positions stretched by
**24/16 = 1.5×** relative to how the checkpoints were trained. We traced this while
investigating consistently odd motion in default Video2World outputs.

Consequences with the current defaults:

- Every default `Cosmos2VideoToWorldPipeline` run diverges from the reference implementation.
- The `fps` argument changes the generated content in diffusers, whereas natively it only
affects the exported container framerate.

Not affected:

- `Cosmos2TextToImagePipeline` — single frame, so the temporal position is 0 either way.
- `Cosmos2_5_PredictBasePipeline` / `Cosmos2_5_TransferPipeline` — they don't take an `fps`
argument, so the transformer sees `fps=None` and uses raw positions, matching the native
configs (which also disable the modulation).
- The Cosmos-Predict1 pipelines are out of scope here; this report is only about the
Cosmos-Predict2 checkpoints.

### Suggested fix

Either of:

1. Port `enable_fps_modulation` into `CosmosTransformer3DModel`'s config (default `False` for
the converted Cosmos-Predict2 checkpoints), mirroring the reference nets — the faithful fix; or
2. Stop passing `fps` to the transformer in `Cosmos2VideoToWorldPipeline` (or change the
default to `fps=24`, which makes the scale factor 1), keeping `fps` for video export only.

### Reproduction

```python
import torch
from diffusers import Cosmos2VideoToWorldPipeline
from diffusers.utils import load_image

pipe = Cosmos2VideoToWorldPipeline.from_pretrained(
"nvidia/Cosmos-Predict2-2B-Video2World", torch_dtype=torch.bfloat16
).to("cuda")

image = load_image(
"https://huggingface.co/datasets/huggingface/documentation-images/resolve/main/diffusers/yellow-scrubber.png"
)

# identical seeds, only fps differs — natively these would be identical, here they are not
common = dict(image=image, prompt="a yellow scrubber cleaning a plate")
video_16 = pipe(**common, fps=16, generator=torch.Generator().manual_seed(1)).frames[0]
video_24 = pipe(**common, fps=24, generator=torch.Generator().manual_seed(1)).frames[0]
# video_24 corresponds to the un-modulated (native) temporal RoPE; video_16 is the default
```

### Logs

```shell

```

### System Info

- diffusers: main (also reproduces on 0.38.0)
- the referenced code is identical on current `main` (line links above)

### Who can help?

@a-r-r-o-w @yiyixuxu

Start in src/diffusers/models/transformers/transformer_cosmos.py and src/diffusers/pipelines/cosmos/pipeline_cosmos2_video2world.py, then compare the referenced Cosmos-Predict2 RoPE and network config files. Reproduce the same-seed fps=16 and fps=24 runs, and verify that the Video2World checkpoints use unmodulated temporal positions while fps remains relevant only to video export.

Rédigé par le modèle d'indexation à partir du texte de l'issue.