Migrate qwenimage pipeline docstrings from torch_dtype to dtype

[iMountTai]

What does this PR do?

Follow-up to #14205, which deprecated torch_dtype in favor of dtype (following transformers). This PR updates all qwenimage pipeline docstring examples to use dtype= instead of the deprecated torch_dtype=, so the documentation no longer demonstrates the deprecated usage.

Pure docstring change — no behavior change. Verified that both pipeline from_pretrained and model from_pretrained accept dtype=.

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Who can review?

@yiyixuxu @dg845

[github-actions]

Hi @iMountTai, thanks for the PR! It does not appear to link an issue it fixes. If this PR addresses an existing issue, please add a closing keyword (e.g. Fixes #1234) to the PR description so the issue is linked. See the contribution guide for more details. If this PR intentionally does not fix a tracked issue, a maintainer can add the no-issue-needed label to silence this reminder.

[ErenAta16]

qwenimage/ comes out clean — grep -rn "torch_dtype" src/diffusers/pipelines/qwenimage/ returns nothing with this applied. The rest of the tree hasn't moved, though:

location	remaining torch_dtype=torch.
src/diffusers/pipelines/ (other families)	358 occurrences across 240 files
docs/	1075

A few of the neighbours, so it's clear this isn't a long tail of oddities:

src/diffusers/pipelines/flux2/pipeline_flux2.py:50
src/diffusers/pipelines/hunyuandit/pipeline_hunyuandit.py:52
src/diffusers/pipelines/helios/pipeline_helios.py:57,58

Both spellings still work — DiffusionPipeline.from_pretrained pops each and resolves them at pipeline_utils.py:785-787:

torch_dtype = kwargs.pop("torch_dtype", None)
dtype = kwargs.pop("dtype", None)
torch_dtype = dtype if dtype is not None else torch_dtype

so nothing breaks either way, and torch_dtype doesn't emit a deprecation warning from diffusers itself. The only place the tree treats it as legacy is pipeline_loading_utils.py:856, and that's about transformers ≥ 4.56 warning on its own side.

So this reads as the first slice of a rename that the docstrings will otherwise stay split on — half the examples using one spelling and half the other is worse for a reader than either spelling consistently. Worth saying in the PR whether more families are coming, or whether a single mechanical pass over src/diffusers/pipelines/ and docs/ is the intent.

Counted on diffusers @ 9f6fc2c with the PR applied.