version-compat CI: fake CPU training runs for SFT/GRPO/DPO (#6965)

* version-compat CI: fake CPU training runs for SFT/GRPO/DPO

Adds a runtime layer on top of the patch-run canary: actually runs
trainer.train() for a couple of steps on a CPU-only runner under the CUDA
spoof, wrapping a plain tiny HF model in the Unsloth-patched trainer. Exercises
the real train() loop (collation, generation, the injected
_get_per_token_logps_and_entropies, loss, backward, optimizer) so a TRL or
transformers change that breaks the loop at runtime -- not just the source
structure -- surfaces here. No GPU, no meaningful numerics.

Needs a chain of small CPU shims (eager torch.compile, dynamo suppress, cuda
tensor-alloc redirect to CPU, model.for_training/for_inference equivalents)
documented inline. Does not exercise Unsloth's Triton/GPU kernels (CPU can't).

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

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

* cpu fake-train: force adamw_torch + disable dynamo for CPU runner

On a real CPU-build torch runner (GitHub CI) two things bit that a CUDA-build
torch with GPUs hidden masked locally:

- The default optimizer is adamw_8bit (bitsandbytes), whose is_on_gpu() check
  dies on CPU tensors. Force optim=adamw_torch in all three configs.
- import unsloth reinstalls the real torch.compile over the eager passthrough,
  so the GRPO hot path (chunked_selective_log_softmax) actually compiles and
  inductor picks the spoofed CUDA device, crashing on device props
  (gcnArchName). Re-apply the eager passthrough after import and flip
  torch._dynamo.config.disable so every @torch.compile runs eager at call time.

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

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

* cpu fake-train: write checkpoints under pytest tmp_path

Use pytest's tmp_path for each trainer's output_dir instead of a hardcoded
relative temp/ci_* path, so a local pytest run does not leave untracked dirs in
the repo tree and the tests are CWD-independent.

* version-compat CI: disable dynamo at process level for the fake-run job

Set TORCHDYNAMO_DISABLE / TORCH_COMPILE_DISABLE in the fake-run step env so
dynamo/inductor is off before conftest.py's early import unsloth, not only via
the per-test runtime shim. Defense in depth on the GPU-less runner: the GRPO
hot path never compiles regardless of when its functions were decorated.

---------

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

.github/workflows/version-compat-ci.yml

@@ -339,12 +339,18 @@ jobs:
         env:
           UNSLOTH_IS_PRESENT: '1'
           UNSLOTH_COMPILE_DISABLE: '1'
+          # Disable dynamo/inductor at the process level, before conftest.py's early
+          # `import unsloth`, so the GRPO hot path never compiles on the GPU-less runner
+          # (defense in depth; the CPU fake-train also flips this at runtime).
+          TORCHDYNAMO_DISABLE: '1'
+          TORCH_COMPILE_DISABLE: '1'
           PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION: python
         run: |
           cd unsloth
           python -c "import trl; print('Resolved TRL', trl.__version__)"
           PYTHONPATH=. python -m pytest \
             tests/version_compat/test_trl_grpo_fake_run.py \
+            tests/version_compat/test_trl_fake_train_cpu.py \
             -v --tb=short
       # `main` is scheduled/dispatch-only so PR jobs stay fast and a bleeding-edge
       # TRL break does not red every PR. github.event_name is valid in a step if.
@@ -353,6 +359,8 @@ jobs:
         env:
           UNSLOTH_IS_PRESENT: '1'
           UNSLOTH_COMPILE_DISABLE: '1'
+          TORCHDYNAMO_DISABLE: '1'
+          TORCH_COMPILE_DISABLE: '1'
           PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION: python
         run: |
           pip install --upgrade "git+https://github.com/huggingface/trl"
@@ -360,6 +368,7 @@ jobs:
           python -c "import trl; print('Resolved TRL', trl.__version__)"
           PYTHONPATH=. python -m pytest \
             tests/version_compat/test_trl_grpo_fake_run.py \
+            tests/version_compat/test_trl_fake_train_cpu.py \
             -v --tb=short
 
   # Daily-only: same suites but with --strict on importable upstream