Tests + CI guard: batched left-padded generation can never silently regress again (#1066, #3699) (#6145)

* Add regression guard for batched left-padded generation (#1066, #3699)

Three layers of tests plus a path-filtered CI workflow so the left-padding
position_ids / attention-mask bug class cannot silently return:

- tests/utils/test_prepare_inputs_ast_guard.py: import-free AST checks on
  _fast_prepare_inputs_for_generation (cumsum-from-mask branch present,
  cache_position only as fallback, no mask truncation, model families wired)
- tests/utils/test_prepare_inputs_leftpad.py: CPU behavioral unit test with
  synthetic left-padded masks and fake caches; exact expected position_ids
  for prefill and cached decode
- tests/utils/test_batched_leftpad_generation_gpu.py: optional GPU e2e,
  solo vs batched prefix match, skipped without CUDA
- .github/workflows/batch-inference-guard.yml: ubuntu-latest CPU job running
  the two deterministic layers on PRs touching unsloth/models/**

Validated: all pass on main; both CPU layers fail at 6d0f8643~1 (pre #4100)
and at 332eabf3~1 (pre #2216), reproducing the historical bug signatures.

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

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* Cite staging proof in batch-inference-guard header (staging-2 PRs 170/171)

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

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* Fold left-padding guard into consolidated Core CI; merge AST + behavioral tests

No new workflow and no new CI job: the guard now runs as one HARD GATE step
inside consolidated-tests-ci.yml, right after the callback signature drift
detector, where the CPU torch stack is already installed. The AST structural
checks and the behavioral unit tests live in a single file
(tests/utils/test_prepare_inputs_leftpad.py); the AST layer stays stdlib-only
with unsloth imported lazily inside the behavioral tests, so import breakage
cannot mask the structural checks.

Revalidated after the merge: 11 assertions pass on main, 8 fail at
6d0f8643~1 (pre #4100).

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

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* Update staging proof reference for consolidated gate (PRs 170/172)

---------

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

.github/workflows/consolidated-tests-ci.yml

@@ -333,6 +333,20 @@ jobs:
         run: |
           python -m pytest -v --tb=short tests/test_callback_signature_drift.py
 
+      - name: batched left-padding generation guard (HARD GATE)
+        # Guards _fast_prepare_inputs_for_generation against the bug class of
+        # issues #1066 / #3699: position_ids taken from cache_position (which
+        # counts left-pad tokens) or the 2D attention mask truncated to its
+        # last column. Both shipped in cc4c5d77 and were fixed by #2216 and
+        # #4100; nothing tested this path, so each regression reached users.
+        # Layer 1 in the file is stdlib-ast-only (survives unsloth import
+        # breakage), layer 2 calls the real function on CPU via the
+        # tests/conftest.py CUDA spoof. Validated to fail on the pre-#2216
+        # and pre-#4100 code states; staging proof on GPU-less runners:
+        # danielhanchen/unsloth-staging-2 PR 170 (green, gate passed in all combos) / PR 172 (red, gate failed in all combos).
+        run: |
+          python -m pytest -v --tb=short tests/utils/test_prepare_inputs_leftpad.py
+
       - name: unsloth Bucket-A — CPU tests not in Repo tests (CPU)
         # CPU tests across 6 files under tests/saving/, tests/utils/, tests/python/
         # that Repo tests (CPU) --ignores. AST/protobuf/regex plus tiny CPU model