mypy: Make Policies module MyPy‑compliant (#1719, #1720)

[akacmazz]

What this does

• Addresses Make LeRobot mypy compliant #1719; fixes Ensure the policy module passes MyPy type checks #1720. (🛠️ Tooling)
• Policies typing:
  • ACT: type‑safe activation helper; move batch tensors to the module device before normalization for consistent
    device semantics.
  • SAC: constructor accepts None to match tests (base class raises the expected error); forward returns a typed dict.
  • Config: PreTrainedConfig now respects LEROBOT_TEST_DEVICE for deterministic device selection in CI/local runs.
• Tooling (pre‑commit): enable a scoped MyPy run via the hook (uses pyproject.toml, installs types, ignores missing
  imports in the hook venv to avoid heavy deps). No global behavior change.

How it was tested

• Static checks:
  • pre-commit run -a → all hooks green (ruff, typos, bandit, mypy).
  • mypy → success on the policies scope configured in pyproject.toml.
• Tests (CPU to keep runs deterministic and lightweight):
  • SAC: PYTHONPATH=src pytest -q tests/policies -k sac
  • ACT back‑compat (uses small artifacts): PYTHONPATH=src pytest -q tests/policies -k 'act and
    backward_compatibility'
• Notes:
  • Set local HF caches to avoid permission issues and reduce flakiness.
  • On low‑memory machines, split ACT/SAC runs to avoid OOM.

How to checkout & try? (for the reviewer)

From repo root

export LEROBOT_TEST_DEVICE=cpu
mkdir -p .cache/hf_datasets .cache/hf_home
export HF_DATASETS_CACHE=$(pwd)/.cache/hf_datasets
export HF_HOME=$(pwd)/.cache/hf_home

Static checks

pre-commit run -a
mypy

SAC tests

PYTHONPATH=src pytest -q tests/policies -k sac

ACT back-compat tests

PYTHONPATH=src pytest -q tests/policies -k 'act and backward_compatibility'
If you’d like me to extend MyPy coverage to the next module (e.g., Datasets) in a follow‑up PR, I can prepare the scoped
config and initial fixes similarly.

cc @CadeRemi @AdilZouitine for review

[AdilZouitine]

Hey thank you for your contribution, I started the review for Act policy 😄
I will do the next policy later 🤗

[AdilZouitine]

Hey we have merged a big pr #1452. Could you rebase this branch please ? I will finish the review 😄

[imstevenpmwork]

Hello @akacmazz

Can you ping me whenever this is ready for a review ? Feel free to ping me in discord
Sorry for the back and forth, thanks !

[akacmazz]

Hey @imstevenpmwork, Just wanted to let you know that my PR for Issue #1720 (MyPy compliance for policies module) is ready for review:

#2295

Thank you.