[model] fix: import AutoModelForCausalLMWithValueHead from trl.experimental.ppo in utils/model.py - #7625

verl-project#7625 added an import block missing the blank line after the
`from transformers import PreTrainedModel` import, which fails
`ruff format` in the pre-commit `--all-files` CI run.

…mental.ppo in utils/model.py (verl-project#7625)

Fixes verl-project#5690

## Motivation

Fixes the remaining gap of
[verl-project#5690](verl-project#5690)
(`verl cannot import name 'AutoModelForCausalLMWithValueHead' from
'trl'`).

PR [verl-project#6539](verl-project#6539) fixed the
import for
`trl >= 0.29` only in `verl/models/transformers/monkey_patch.py`, but
the same
top-level import still exists in **two places** in
`verl/utils/model.py`, which
are hit when the FSDP engine loads a value-head critic
(`verl/workers/engine/fsdp/transformer_impl.py:272` ->
`load_valuehead_model`):

- `verl/utils/model.py:611` (`patch_valuehead_model`)
- `verl/utils/model.py:657` (`load_valuehead_model`)

With `trl >= 0.29` (e.g. the latest `trl==1.12.0`, which removed the
top-level
class in huggingface/trl#5044), PPO/RLHF training with a value-head
critic on
the FSDP engine crashes with `ImportError`. This also blocks trl
dependency
bumps (see closed dependabot PR verl-project#7478).

Verified on `trl==1.12.0`:

```
>>> from trl import AutoModelForCausalLMWithValueHead
ImportError: cannot import name 'AutoModelForCausalLMWithValueHead' from 'trl'
>>> from trl.experimental.ppo import AutoModelForCausalLMWithValueHead
OK  # trl.experimental.ppo.modeling_value_head.AutoModelForCausalLMWithValueHead
```

## Change

Mirror the existing pattern in
`verl/models/transformers/monkey_patch.py:351-353`
at both sites in `verl/utils/model.py`:

```python
try:
    from trl.experimental.ppo import AutoModelForCausalLMWithValueHead  # type: ignore
except ImportError:
    from trl import AutoModelForCausalLMWithValueHead  # type: ignore
```

- `trl >= 0.29` (experimental path exists): uses `trl.experimental.ppo`.
- `trl < 0.29` (e.g. the fsdp/megatron extra pin `trl==0.27.0`):
experimental
import fails and falls back to the top-level import — unchanged
behavior.

## Why this is not duplicating an existing PR

Checked `gh pr list --repo verl-project/verl --state open --search
"trl"` and
`--search "valuehead OR value_head OR
AutoModelForCausalLMWithValueHead"`:
no open PR touches `verl/utils/model.py` imports. verl-project#6539 (merged) covered
only
`monkey_patch.py`; this PR completes the same fix for the FSDP valuehead
path
that verl-project#6539 missed.

## Tests

- `pip install trl==1.12.0` in a scratch env; both import paths verified
as
  above (top-level fails, `trl.experimental.ppo` works).
- Existing CPU unit tests unaffected:
`python -m pytest
tests/workers/rollout/test_chunked_prefill_batched_tokens_on_cpu.py -v`
(6 passed)
and the wider CPU suite (`cpu_tests_tf510.log`: 613 passed) were used as
a
  regression baseline on this machine.
- (Optional, pending reviewer preference) a unit test asserting the
import
  resolves under both trl layouts via `sys.modules` stubbing.

## AI assistance disclosure

Prepared with AI assistance (draft + verification). The submitting human
has
reviewed the design, will review every changed line, and has run the
verification commands above before submitting.

Co-authored-by: LiRunGuo <[email protected]>