fix: preserve batch-major LAM action ordering

cosmos_predict2/_src/predict2/models/text2world_model_rectified_flow.py

@@ -865,7 +865,10 @@ def forward(self, data_batch: dict[str, torch.Tensor]) -> tuple[dict[str, torch.
         with torch.no_grad():
             outputs = self.lam.lam(lam_input)
         latent_action = outputs["z_rep"].squeeze().to(data_batch["action"].dtype).detach()
-        latent_action = rearrange(latent_action, "(t b) d -> b t d", t=data_batch["action"].shape[1])
+        # ``lam_video`` is flattened in batch-major order (b, p) above.  Keep
+        # that order when restoring the per-sample action sequence; using
+        # (t, b) here silently mixes action tokens between batch elements.
+        latent_action = rearrange(latent_action, "(b t) d -> b t d", t=data_batch["action"].shape[1])
         data_batch["action"][:, :, -32:] = data_batch["action"][:, :, -32:] * latent_action
 
         # Get the input data to noise and denoise~(image, video) and the corresponding conditioner.

tests/test_lam_batch_order.py

@@ -0,0 +1,82 @@
+# SPDX-FileCopyrightText: Copyright (c) 2025 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+# SPDX-License-Identifier: Apache-2.0
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Regression tests for the LAM action batch/time layout.
+
+The full DreamDojo model needs CUDA-only dependencies, so this test keeps the
+fixture dependency-free and checks the layout contract directly from the
+source.  The unique token fixture makes a batch/time transpose observable even
+when tensor shapes remain unchanged.
+"""
+
+import ast
+from pathlib import Path
+
+import torch
+
+
+_MODEL_SOURCE = (
+    Path(__file__).parents[1]
+    / "cosmos_predict2"
+    / "_src"
+    / "predict2"
+    / "models"
+    / "text2world_model_rectified_flow.py"
+)
+
+
+def _forward_rearrange_patterns() -> list[str]:
+    tree = ast.parse(_MODEL_SOURCE.read_text(encoding="utf-8"))
+    forward = next(
+        node
+        for node in ast.walk(tree)
+        if isinstance(node, ast.FunctionDef) and node.name == "forward"
+    )
+    patterns = []
+    for node in ast.walk(forward):
+        if not isinstance(node, ast.Call) or not isinstance(node.func, ast.Name):
+            continue
+        if node.func.id != "rearrange" or len(node.args) < 2:
+            continue
+        pattern = node.args[1]
+        if isinstance(pattern, ast.Constant) and isinstance(pattern.value, str):
+            patterns.append(pattern.value)
+    return patterns
+
+
+def test_lam_restore_uses_batch_major_inverse():
+    """The inverse must match ``(b p)`` flattening used for LAM videos."""
+
+    patterns = _forward_rearrange_patterns()
+    assert "b (p t) h w c -> (b p) t h w c" in patterns
+    assert "(b t) d -> b t d" in patterns
+    assert "(t b) d -> b t d" not in patterns
+
+
+def test_batch_major_restore_keeps_each_sample_sequence_intact():
+    batch_size, action_steps, width = 3, 4, 2
+    flattened = torch.arange(batch_size * action_steps * width).reshape(
+        batch_size * action_steps, width
+    )
+
+    restored = flattened.reshape(batch_size, action_steps, width)
+    expected = torch.stack(
+        [flattened[s * action_steps : (s + 1) * action_steps] for s in range(batch_size)]
+    )
+
+    torch.testing.assert_close(restored, expected)
+    # A time-major interpretation has the same shape but mixes samples.
+    time_major = flattened.reshape(action_steps, batch_size, width).transpose(0, 1)
+    assert not torch.equal(restored, time_major)