Validate DDPM generator list length against batch size

[X-mo7]

What does this PR do?

This PR adds validation in DDPMPipeline to ensure that when a list of torch.Generators is provided, its length matches the requested batch_size. Before, DDPMPipeline could accept mismatched generator lists (they would silently succeed or fail later with an error depending on the list length). The new validation follows the existing behavior in the sibling DDIMPipeline and raises a clear ValueError before noise generation. A focused regression test was added to cover this case. It addresses Issue 2 in #13649.

Tests:
.venv/bin/pytest tests/pipelines/ddpm/test_ddpm.py::TestDDPMPipeline::test_generator_list_batch_size_mismatch_raises -q

1 passed

.venv/bin/pytest tests/pipelines/ddpm/test_ddpm.py -q

15 passed, 23 skipped

Formatting/style check:
PATH=.venv/bin:$PATH make modified_only_fixup

All checks passed; 2 files left unchanged

Help from AI:
Codex was used to assist with investigating the issue, implementing the change, and reviewing the final diff.

Final self-review findings:

• The DDPM validation matches the existing DDIM validation in wording and structure
• The validation is placed before the initial randn_tensor(...) call
• No shared helper or unrelated refactor is needed
• The regression test reuses the existing DDPM test helpers and covers the reported mismatch
• No unrelated changes or outstanding self-review findings remain

Before submitting

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• Was this discussed/approved via a GitHub issue or the forum? Please add a link to it if that's the case : ddpm model/pipeline review #13649 (comment)
• Did you make sure to update the documentation with your changes? Here are the
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  here are tips on formatting docstrings.
• Did you write any new necessary tests?
• Are you the author (or part of the team) of the model/pipeline (only applicable for model/pipeline related PRs)?

Who can review?

cc @yiyixuxu @dg845

[github-actions]

Hi @X-mo7, 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.

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[X-mo7]

#13649 tracks multiple separate findings. This PR specifically addresses Issue 2 only, so I avoided using Fixes #13649 since that would close the whole issue while other findings remain open. I'm happy to adjust the tracking if a maintainer prefers a different approach.