lerobot/tests/policies/pi052/test_pi052_fast_action_loss.py

#!/usr/bin/env python

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"""Regression tests for PI052 FAST action-code supervision."""

from types import SimpleNamespace

import pytest
import torch
from torch import nn
from torch.nn import functional as F  # noqa: N812

pytest.importorskip("transformers")
pytest.importorskip("liger_kernel")

from lerobot.policies.pi052.modeling_pi052 import PI052Policy, _fast_lin_ce  # noqa: E402
from lerobot.policies.pi052.processor_pi052 import make_pi052_pre_post_processors  # noqa: E402


def _fast_ce(logits, action_tokens, action_code_mask, predict_actions_t):
    """Adapter: ``_fast_lin_ce`` is Liger-fused (hidden @ lm_head_weightᵀ).

    Feeding an identity ``lm_head_weight`` makes the computed logits equal the
    provided ``logits``, so these regression tests exercise the masking/gating
    logic exactly as before the fused-CE refactor. Liger's Triton kernel is
    GPU-only, so inputs are moved to CUDA and the loss is returned on CPU
    (keeping grad flowing back to the CPU ``logits`` leaf).
    """
    if not torch.cuda.is_available():
        pytest.skip("Liger fused CE requires CUDA")
    vocab_size = logits.shape[-1]
    eye = torch.eye(vocab_size, dtype=logits.dtype, device="cuda")
    predict = predict_actions_t.cuda() if predict_actions_t is not None else None
    loss = _fast_lin_ce(logits.cuda(), eye, action_tokens.cuda(), action_code_mask.cuda(), predict)
    return loss.cpu()


def test_fast_ce_supervises_only_discrete_action_codes():
    """Wrapper tokens can be wrong without affecting the FAST action-code loss."""
    vocab_size = 8
    action_tokens = torch.tensor([[1, 2, 3, 4, 5, 0]])
    action_code_mask = torch.tensor([[False, False, True, True, False, False]])

    logits = torch.zeros(1, action_tokens.shape[1], vocab_size)
    # Deliberately bad wrapper-token predictions. These should be ignored.
    logits[0, 0, 7] = 10.0  # target would be token 2
    logits[0, 3, 7] = 10.0  # target would be delimiter token 5
    # Correct action-code predictions: hidden t predicts target t + 1.
    logits[0, 1, 3] = 10.0
    logits[0, 2, 4] = 10.0

    loss = _fast_ce(logits, action_tokens, action_code_mask, predict_actions_t=None)
    expected = F.cross_entropy(
        torch.stack([logits[0, 1], logits[0, 2]]),
        torch.tensor([3, 4]),
        reduction="mean",
    )

    # Allow the fused GPU kernel's ~1e-7 difference on small losses.
    assert torch.allclose(loss, expected, atol=1e-5, rtol=1e-3)


def test_fast_ce_masks_non_action_samples():
    """Recipe samples with predict_actions=False do not contribute FAST loss."""
    vocab_size = 8
    action_tokens = torch.tensor([[1, 2, 3, 4], [1, 2, 5, 6]])
    action_code_mask = torch.tensor([[False, False, True, True], [False, False, True, True]])
    predict_actions = torch.tensor([True, False])

    logits = torch.zeros(2, action_tokens.shape[1], vocab_size)
    logits[0, 1, 3] = 10.0
    logits[0, 2, 4] = 10.0
    # Bad predictions in the masked sample should not matter.
    logits[1, 1, 7] = 10.0
    logits[1, 2, 7] = 10.0

    loss = _fast_ce(logits, action_tokens, action_code_mask, predict_actions)
    expected = F.cross_entropy(
        torch.stack([logits[0, 1], logits[0, 2]]),
        torch.tensor([3, 4]),
        reduction="mean",
    )

    # Allow the fused GPU kernel's ~1e-7 difference on small losses.
    assert torch.allclose(loss, expected, atol=1e-5, rtol=1e-3)


def test_fast_ce_returns_zero_when_no_action_code_positions_are_valid():
    logits = torch.randn(2, 4, 8, requires_grad=True)
    action_tokens = torch.tensor([[1, 2, 3, 4], [1, 2, 5, 6]])
    action_code_mask = torch.zeros_like(action_tokens, dtype=torch.bool)

    loss = _fast_ce(logits, action_tokens, action_code_mask, predict_actions_t=None)

    assert loss.item() == 0
    loss.backward()
    assert logits.grad is not None


def test_fast_ce_averages_each_action_sample_equally():
    torch.manual_seed(0)
    hidden = torch.randn(2, 5, 8)
    lm_head_weight = torch.eye(8)
    action_tokens = torch.tensor([[1, 2, 0, 0, 0], [1, 3, 4, 5, 6]])
    action_code_mask = torch.tensor([[False, True, False, False, False], [False, True, True, True, True]])

    loss = _fast_lin_ce(
        hidden,
        lm_head_weight,
        action_tokens,
        action_code_mask,
        predict_actions_t=None,
        reduction="mean",
    )
    per_sample = _fast_lin_ce(
        hidden,
        lm_head_weight,
        action_tokens,
        action_code_mask,
        predict_actions_t=None,
        reduction="none",
    )

    assert torch.allclose(loss, per_sample.mean())


def test_pi052_rejects_fast_loss_without_recipe():
    config = SimpleNamespace(recipe_path=None, enable_fast_action_loss=True)

    with pytest.raises(ValueError, match="recipe_path"):
        make_pi052_pre_post_processors(config)


def test_pi052_rejects_missing_fast_batch_keys():
    policy = PI052Policy.__new__(PI052Policy)
    nn.Module.__init__(policy)
    policy.config = SimpleNamespace(
        enable_fast_action_loss=True,
        fast_action_loss_weight=1.0,
        flow_loss_weight=0.0,
        text_loss_weight=1.0,
    )
    batch = {
        "text_labels": torch.tensor([[1, 2]]),
        "predict_actions": torch.tensor([True]),
    }

    with pytest.raises(ValueError, match="FAST action loss is enabled"):
        policy.forward(batch)