fix(wbc): drop wrong-family default checkpoint; actionable error + SONIC-stack detection

[cagataycali]

Problem

create_policy("wbc") with no checkpoint defaulted to the HuggingFace repo nvidia/GEAR-SONIC. That repo ships the SONIC VLA inference stack (model_encoder.onnx, model_decoder.onnx, planner_sonic.onnx, low_latency/*) — not the decoupled-WBC Balance/Walk ONNX policies WBCPolicy actually loads (policy.onnx / walk_policy.onnx).

The decoupled-WBC G1 weights live only in the NVlabs/GR00T-WholeBodyControl git-LFS tree at decoupled_wbc/sim2mujoco/resources/robots/g1/policy/ (GR00T-WholeBodyControl-Balance.onnx + GR00T-WholeBodyControl-Walk.onnx), not as a standalone HF repo.

So a bare create_policy("wbc") silently downloaded ~MB of the wrong model family, then failed with a confusing WBCPolicy main ONNX checkpoint not found — whose own hint pointed back at nvidia/GEAR-SONIC, sending the user in a circle.

Change

• Remove the _DEFAULT_HF_REPO = "nvidia/GEAR-SONIC" default. No weights are bundled and there is no working default repo to guess, so a missing checkpoint now raises an actionable error before any network call, pointing at the correct NVlabs git-LFS source.
• Add _reject_sonic_inference_stack(): if a checkpoint directory holds the SONIC encoder/decoder/planner ONNX but no policy.onnx, raise a clear error up front explaining it is the wrong family and where to obtain the Balance/Walk weights (covers the case where a user explicitly passes the SONIC checkpoint). A dir that colocates both a SONIC marker and policy.onnx is left alone.
• Centralize the not-found message in _checkpoint_not_found_message() with distinct text for the no-checkpoint vs. resolved-but-missing cases.
• Docs: docs/policies/wbc.md now explains there is no default download and gives the exact git clone + copy steps from the NVlabs LFS tree; clarifies that nvidia/GEAR-SONIC is the SONIC inference stack, not this family. Updated docs/training/vla_workflow.md + example placeholder paths to a neutral grootwbc-g1 dir name.

Behavior after fix

>>> create_policy("wbc")
RuntimeError: WBCPolicy requires a checkpoint but none was provided. No weights are
bundled. Obtain GR00T-WholeBodyControl-Balance.onnx and GR00T-WholeBodyControl-Walk.onnx
from the NVlabs/GR00T-WholeBodyControl git-LFS tree
(decoupled_wbc/sim2mujoco/resources/robots/g1/policy/), place them in a directory as
policy.onnx + walk_policy.onnx, and pass checkpoint='/path/to/that/dir'. Note: the
HuggingFace repo nvidia/GEAR-SONIC is the SONIC VLA inference stack
(encoder/decoder/planner), not this decoupled-WBC Balance/Walk family.

No network call is made for the no-checkpoint path (verified by the regression test, which wires snapshot_download to assert-raise if reached).

Tests

New TestDefaultCheckpoint class in tests/policies/wbc/test_policy.py:

• test_no_default_hf_repo_constant — the wrong-family default constant is gone.
• test_create_policy_wbc_without_checkpoint_raises_actionable_error — bare create_policy("wbc") raises the actionable error and attempts no download.
• test_no_checkpoint_message_points_at_nvlabs_lfs — message names the LFS path + warns GEAR-SONIC is the wrong family.
• test_sonic_inference_stack_dir_rejected / test_sonic_marker_with_policy_onnx_is_allowed / test_reject_sonic_noops_on_none_and_files — SONIC-stack detection.
• test_explicit_checkpoint_dir_still_works — explicit local checkpoint dir happy path unchanged.

These fail on pre-fix code (reproducing the nvidia/GEAR-SONIC download) and pass after.

Gate

• ruff check strands_robots tests tests_integ: clean
• ruff format --check: clean
• mypy strands_robots tests tests_integ: Success, no issues in 564 source files
• MUJOCO_GL=egl pytest tests/policies/wbc/: 109 passed (7 new)

No behavior change to a loaded gait — this is a loading-path correctness + UX fix, so a rollout video is not applicable; the before/after error reproduction above is the relevant proof.

[yinsong1986]

Summary

This PR removes the wrong-family default checkpoint (_DEFAULT_HF_REPO = "nvidia/GEAR-SONIC") from WBCPolicy. Previously a bare create_policy("wbc") silently downloaded the SONIC VLA inference stack (encoder/decoder/planner ONNX) instead of the decoupled-WBC Balance/Walk policies the provider actually loads, then failed with a confusing policy.onnx not found whose hint pointed back at the same wrong repo. Now a missing checkpoint raises an actionable RuntimeError before any network call, naming the correct NVlabs/GR00T-WholeBodyControl git-LFS source. A new _reject_sonic_inference_stack() guard rejects an explicitly-passed SONIC-stack directory up front (while leaving a dir that colocates a SONIC marker with policy.onnx alone), and the not-found message is centralized in _checkpoint_not_found_message(). Docs and example placeholder paths are updated to a neutral grootwbc-g1 dir name.

Traced the no-checkpoint path: checkpoint=None -> _maybe_download_checkpoint(None) returns None (no network), _reject_sonic_inference_stack(None) short-circuits, and the resolver raises the centralized no-checkpoint message. The default-behavior change is loud (explicit RuntimeError, not a silent coercion), the removed constant is private (_-prefixed, not in __all__), and the SONIC-detection helper handles the None/file/dir cases without a crash. No security surface, no wire-format/schema/public-signature changes, no host paths or emojis in the new strings. Verdict: no blocking concerns.

What's good

• Fails fast with an actionable error before the network call, matching AGENTS.md #5/#6 (raise on fatal, no silent wrong-family fetch) and the loud-default-change pattern (AGENTS.md PR #86 > Safety Defaults).
• Regression tests pin the behavior: snapshot_download is wired to assert-raise if reached, and the SONIC-detection edge cases (None, a file, marker-with-policy.onnx) each have a dedicated test.
• Scope-disciplined: a loading-path correctness + UX fix with matching doc/example updates, no incidental behavior change to a loaded gait.