fix(hf): pass return_dict=True to apply_chat_template

[ajbozarth]

Issue

Fixes #

Description

Fixes a latent bug introduced in #418 (transformers 5 bump) where LocalHFBackend.generate_from_context indexes a raw torch.Tensor with a string key.

apply_chat_template(..., return_tensors="pt") returns a 2-D torch.Tensor, not a dict. The standard-generation call site at huggingface.py:1119-1120 then does:

inputs=input_ids["input_ids"],
attention_mask=input_ids["attention_mask"],

Per the warning at pytorch/csrc/autograd/python_variable_indexing.cpp:353, in torch ≥ 2.9 this expression is interpreted as tensor[torch.tensor(seq)] — fancy indexing with the codepoints of "input_ids" against a 2-D tensor — which raises:

IndexError: too many indices for tensor of dimension 2
mellea/backends/huggingface.py:1119: IndexError

Reproduces locally on macOS Apple Silicon (torch 2.11.0, both CPU and MPS).

The fix is one line: pass return_dict=True to apply_chat_template so the dict access (and the .to(self._device) chain) operate on a BatchEncoding.

Why return_dict=True and not "pass the bare tensor"

Two producers feed the local input_ids variable into the same downstream processing() and post_processing() consumers:

1. The merged-cache helper (_compute_input_ids_with_kv_cache_smashing, line 831) — returns a bare torch.Tensor.
2. The standard-generation site (line 1054) — apply_chat_template(...) — should return a BatchEncoding.

The consumers already handle both shapes via isinstance(input_ids, torch.Tensor) ternaries at lines 1200, 1273, 1305 (Tensor branch for the merged-cache producer, else input_ids["input_ids"] for the dict producer). The dict branch is only reachable if the standard-generation producer returns a dict, so adding return_dict=True matches what consumers were coded for. The alternative would also require deleting all three dict branches and synthesizing an attention_mask.

Why this slipped through

• PR CI doesn't catch it. test_telemetry/test_metrics_backend.py::test_huggingface_token_metrics_integration is gated by @require_gpu(min_vram_gb=8) (test/predicates.py:91). GitHub Actions runners have neither CUDA nor MPS, so the test is skipped on every PR run.

• GPU nightly runs do not fail on it. Confirmed against the most recent main-branch nightly log (Linux + CUDA, same torch==2.11.0 per uv.lock): both parametrizations of the test ran end-to-end (the transformers max_length warning fires from inside model.generate(), past the broken site) and neither appears in the failure list. The pytorch deprecation warning that fires every run locally is completely absent from the cluster log.

  Same mellea code, same indexing expression, same torch version string, same Python, same filterwarnings config — but different platform wheels (uv.lock ships ~530MB Linux x86_64 with CUDA vs. ~80MB macOS ARM64; distinct compiled binaries). Canonical pytorch source for treatSequenceAsTuple is platform-agnostic and should warn on every platform; the cluster wheel evidently diverges from that. Mechanism uninvestigated — not blocking.

What didn't change

• Public API, signatures, behavior: unchanged.
• Merged-cache path: still returns a raw Tensor; the three downstream ternaries continue to handle both producers.
• Other apply_chat_template call sites: only line 1054 was buggy. The KV-cache call at line 774 uses tokenize=False (returns a string) and the alora batch path at 1511-1550 already operates on a dict-shaped result.

Testing

• Tests added to the respective file if code was changed
• New code has 100% coverage if code was added
• Ensure existing tests and github automation passes (a maintainer will kick off the github automation when the rest of the PR is populated)

Locally on Apple Silicon (torch 2.11.0, transformers 5.x, MPS):

$ uv run pytest test/telemetry/test_metrics_backend.py::test_huggingface_token_metrics_integration -v
test_huggingface_token_metrics_integration[non-streaming] PASSED
test_huggingface_token_metrics_integration[streaming]     PASSED

The python_variable_indexing.cpp:353 warning is gone. No new test added — test_huggingface_token_metrics_integration already exercises this path; it's just GPU-gated. Will validate on the GPU path post-merge.

Attribution

• AI coding assistants used

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