Respect GC for GRPO (#5269)

* Respect GC for GRPO

* Preserve gradient_checkpointing across post-generate training-mode restores

Two sibling generation paths put the model into inference mode and then
unconditionally restored training with the for_training default, which
re-enabled gradient checkpointing even when the caller had it disabled:

- unsloth/models/rl.py: unsloth_unwrap_model_for_generation, installed
  onto every TRL *_trainer module that exposes unwrap_model_for_generation.
- unsloth/models/llama.py: unsloth_fast_generate, bound onto model.generate.

Snapshot the active gradient_checkpointing state from the model modules
before for_inference clears it, then thread the snapshot through the
matching for_training call. Same one-line restore semantics already used
by prepare_for_training_mode and the GRPO replacement at rl_replacements.py.

The for_training(...) call on each line is preserved; only the kwarg is
added. The pre-existing post-generate guards (the conditional restore in
unsloth_fast_generate and the finally restore in
unsloth_unwrap_model_for_generation) continue to run unchanged.

* Snapshot pre-disable, preserve unsloth smart-GC mode across generation restores

Two follow-ups to the post-generate gradient_checkpointing restore:

1. unsloth/models/rl.py: TRL's _unwrap_model_for_generation calls
   unwrapped_model.gradient_checkpointing_disable() before yielding
   (trl/models/utils.py:124-127 in 0.22.2, 0.27.1, and 1.3.0). The
   previous snapshot was taken inside the with-block and therefore read
   the post-disable state, restoring for_training with
   use_gradient_checkpointing=False even when the caller had it on. Move
   the snapshot above the with-block so it observes the caller's
   pre-disable configuration.

2. unsloth/models/{rl.py,llama.py}: any(getattr(m, "gradient_checkpointing"))
   collapses Unsloth's smart-GC mode value "unsloth" (a documented loader
   default at unsloth/models/_utils.py:212 and unsloth/models/llama.py
   2824/3314, loader.py:248/854) into a plain True. After generation, the
   restore would silently downgrade "unsloth" smart GC to standard HF GC.
   Replace any() with a value-preserving next((v for ... if v), False) so
   the actual mode value survives the round-trip.

The for_training(...) calls on each line are preserved; only the snapshot
expression and its position change. The pre-existing post-generate restore
guards continue to run unchanged.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

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Co-authored-by: Daniel Han <danielhanchen@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>