[generation] Fix misleading synced_gpus warning in continuous batching
Author: Partha-Shankar
Pull request: yes

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# What does this PR do?
This PR fixes a misleading user-facing warning message when calling `generate()` with `synced_gpus=True` in continuous batching mode (e.g. `cache_implementation="paged"`). 
Closes #47276
## Summary
The warning message previously stated:
`synced_gpus is not ignored for continuous batching. Got synced_gpus = True`
This double negative incorrectly implies that `synced_gpus` is active or supported. In fact, `synced_gpus` is discarded during continuous batching execution. The comment directly above the warning also explicitly reads `# others are ignored`.
This PR corrects the warning to state:
`synced_gpus is ignored for continuous batching. Got synced_gpus = True`
## Root Cause
A simple negation typo in the warning string format.
## Solution
Change `"is not ignored"` to `"is ignored"` in the warning message in `src/transformers/generation/utils.py`.
### Why This Is a Standalone Fix
This PR addresses a user-facing negation typo in the warning message logged
when `synced_gpus` is passed during continuous batching. Because continuous
batching ignores `synced_gpus`, the warning mistakenly read "synced_gpus is
not ignored...", which can cause significant confusion for users debugging
distributed setups who believe the parameter is active. As this is a highly
visible, misleading warning specifically affecting the continuous batching
entry point, and a search of the generation codebase confirmed no other
stale warning negation bugs nearby, it is submitted as a targeted, standalone
fix rather than bundled into a broader cleanup.
## Tests Run
- Verified the fix manually and confirmed no regressions via the full existing test suite: `pytest tests/generation/test_continuous_batching.py -v` → 49 passed, 71 skipped, 0 failed.
- Checked code style and formatting via `ruff check` and `ruff format` locally.
### AI-Assisted Tooling & Guidelines Compliance
- **AI Tooling Used**: Google Antigravity was used to review the codebase and
  assist in understanding the continuous batching parameter validation flow.
  The fix was authored directly by the developer.
- **Coordination Link**: huggingface/transformers#47276
- **Duplicate-PR Check**: Searched open and recently-closed pull requests on
  huggingface/transformers for PRs touching the same warning string or file
  (src/transformers/generation/utils.py). No duplicates or similar
  sweeping cleanup PRs were found.
- **Verification & Test Output**:
```bash
  $ ruff check src/transformers/generation/utils.py
  All checks passed!
```
```bash
  $ ruff format --check src/transformers/generation/utils.py
  1 file already formatted
```
```bash
  $ pytest tests/generation/test_continuous_batching.py -v
  ======================= 49 passed, 71 skipped in 12.14s =======================
```

Comments:
Thanks for fixing the nit, please remove the test and I will merge.

Done — removed the test, only the warning-message fix remains. Thanks for the review!

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_47158). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

Hi @remi-or — after updating the branch to sync with main, the PR went from a single failing examples_torch check to multiple failing checks across tests_non_model, tests_processors, and tests_tokenization (multiple shards). This PR still only contains the one-line warning-message change in generation/utils.py, so these failures seem unrelated to the change itself. Would you recommend waiting for CI to stabilize or rerunning the affected checks? Happy to follow your guidance. Thanks!

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### CI recap

**Dashboard:** [View test results in Grafana](https://transformers-ci.lor-e.huggingface.cool/d/pytest-observability-by-pr/pytest-observability-branch?var-pr=47158)
**Latest run:** [29309609583:2](https://github.com/huggingface/transformers/actions/runs/29309609583)
**Result:** `success` | **Jobs:** 15 | **Tests:** 171,053 | **Failures:** 0 | **Duration:** 14h 46m

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