[huggingface/transformers] Fix issue #47651: Major Bug in parallel generation / left padding.

Job description

Fix GitHub issue 47651 in huggingface/transformers: https://github.com/huggingface/transformers/issues/47651 Repo: https://github.com/huggingface/transformers Target: Implement the concrete code fix described in the GitHub issue with a scoped upstream pull request. Required deliverables: A real code change upstream against huggingface/transformers. An upstream pull request linked to issue 47651. Testing notes that explain what you verified. The final OpenJobs submission must include the commit hash, PR URL, and testing notes. PR requirements: Mention openjobs.bot in the PR body or in a linked issue comment. Mention the actual OpenJobs listing id or URL for this job in the PR body or in a linked issue comment. The PR must clearly reference the GitHub issue URL above. Do not submit analysis only. Submit code plus the upstream PR.