Fix Qwen Omni batched text postprocessing
Author: Sunt-ing
Pull request: yes

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# What does this PR do?

Batched text-only `pipeline("any-to-any")` calls with Qwen Omni can return only the first row. The model returns a full `(batch, seq)` tensor for text generation, but the processor always decoded `generated_outputs[0]`. That indexing is correct for tuple/list multimodal outputs where text sequences are the first item; it is wrong for a plain text tensor.

This PR keeps the tuple/list path unchanged and decodes the whole tensor for text-only outputs. The same fix is applied to Qwen2.5-Omni and Qwen3-Omni's matching processor method.

This supersedes the closed PR #47060 with a real `pipeline("any-to-any")` repro.

<details>
<summary>End to end any-to-any pipeline repro</summary>

Environment:

```text
Hardware: AutoDL c4090, CPU-only run with CUDA_VISIBLE_DEVICES=""
Python: 3.12.3
PyTorch: 2.8.0+cu128
Transformers main: 187fda1bb179f5a2e261736bb4d91c292864ca0d
Model: hf-internal-testing/tiny-random-Qwen2_5OmniForConditionalGeneration
Entrypoint: pipeline("any-to-any", model=model, processor=processor)
Extra deps installed: librosa, soundfile, soxr
Command: CUDA_VISIBLE_DEVICES="" PYTHONPATH=src:. python repro.py
```

Script:

```python
import os

os.environ["CUDA_VISIBLE_DEVICES"] = ""

from transformers import AutoProcessor, Qwen2_5OmniForConditionalGeneration, pipeline


MODEL_ID = "hf-internal-testing/tiny-random-Qwen2_5OmniForConditionalGeneration"


processor = AutoProcessor.from_pretrained(MODEL_ID)
model = Qwen2_5OmniForConditionalGeneration.from_pretrained(MODEL_ID).eval()
pipe = pipeline("any-to-any", model=model, processor=processor)

outputs = pipe(
    text=["hello", "world"],
    return_full_text=False,
    generate_kwargs={
        "generation_mode": "text",
        "thinker_do_sample": False,
        "thinker_max_new_tokens": 1,
    },
)

print("PIPE_OUT_LEN", len(outputs))
print("PIPE_OUT", outputs)
if len(outputs) != 2:
    raise SystemExit("expected two outputs for a two-item text batch")
```

Current main:

```text
PIPE_OUT_LEN 1
PIPE_OUT [{'input_text': 'hello', 'generated_text': ' marvel'}]
expected two outputs for a two-item text batch
```

After this PR:

```text
PIPE_OUT_LEN 2
PIPE_OUT [{'input_text': 'hello', 'generated_text': ' marvel'}, {'input_text': 'world', 'generated_text': '_VOLUME'}]
```

Reverting only this patch makes the same script fail again with `PIPE_OUT_LEN 1`.

</details>

<details>
<summary>Checks</summary>

```text
git diff --check
passed
```

</details>

## Code Agent Policy

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## Who can review?

cc @Rocketknight1

Comments:
Looks good! Can you add a quick regression test, and then I'll merge?

Added. Thanks~

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_47197). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

**[For maintainers]** Suggested jobs to run (before merge)

run-slow: qwen2_5_omni, qwen3_omni_moe

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