fix(evo1): infer batch size after normalizing image dims

`_collect_image_batches` read `batch_size = batch[camera_keys[0]].shape[0]`
before normalizing per-camera tensors to `(B, C, H, W)`. For an unbatched
`(C, H, W)` input (which the function tries to support via the `image.dim() == 3`
branch), this picked up the channel count `C` instead of the real batch size,
making the subsequent per-sample loop iterate `C` times and indexing go
out of bounds.

Normalize each camera tensor up-front, then read `batch_size` from the
normalized batch dim. Adds `test_collect_image_batches_handles_unbatched_chw`
covering the regression.

Reported by Copilot review on huggingface/lerobot#3545.

src/lerobot/policies/evo1/modeling_evo1.py

@@ -299,12 +299,10 @@ class EVO1Policy(PreTrainedPolicy):
         camera_keys = self._camera_keys or sorted(key for key in batch if key.startswith(f"{OBS_IMAGES}."))
         if not camera_keys:
             raise ValueError("EVO1 requires at least one visual observation feature.")
-        batch_size = batch[camera_keys[0]].shape[0]
-        image_batches: list[list[Tensor]] = []
-        image_masks = torch.zeros(batch_size, self.config.max_views, dtype=torch.bool)
 
-        for batch_index in range(batch_size):
-            sample_images: list[Tensor] = []
+        # Normalize each camera tensor to (B, C, H, W) up-front so that batch_size is read
+        # from a real batch dim and not from C in the unbatched (C, H, W) case.
+        normalized: dict[str, Tensor] = {}
         for camera_key in camera_keys[: self.config.max_views]:
             image = batch[camera_key]
             if image.dim() == 3:
@@ -315,7 +313,16 @@ class EVO1Policy(PreTrainedPolicy):
                 raise ValueError(
                     f"Unsupported image tensor shape for EVO1: key={camera_key} shape={tuple(image.shape)}"
                 )
-                sample_images.append(image[batch_index].detach().cpu())
+            normalized[camera_key] = image
+
+        batch_size = normalized[camera_keys[0]].shape[0]
+        image_batches: list[list[Tensor]] = []
+        image_masks = torch.zeros(batch_size, self.config.max_views, dtype=torch.bool)
+
+        for batch_index in range(batch_size):
+            sample_images: list[Tensor] = []
+            for camera_key in camera_keys[: self.config.max_views]:
+                sample_images.append(normalized[camera_key][batch_index].detach().cpu())
             if not sample_images:
                 raise ValueError("EVO1 received a batch without any image tensor.")
             while len(sample_images) < self.config.max_views:

tests/policies/evo1/test_evo1.py

@@ -180,6 +180,23 @@ def test_evo1_policy_forward_and_inference_use_batched_embedding(monkeypatch):
     assert selected.shape == (2, ACTION_DIM)
 
 
+def test_collect_image_batches_handles_unbatched_chw(monkeypatch):
+    # Regression for an issue where batch_size was read from shape[0] before normalizing
+    # per-camera tensor dims, so an unbatched (C, H, W) input was treated as batch_size=C.
+    monkeypatch.setattr(modeling_evo1, "EVO1", DummyEVO1)
+    policy = modeling_evo1.EVO1Policy(make_config())
+    batch = {
+        OBS_STATE: torch.randn(1, STATE_DIM),
+        f"{OBS_IMAGES}.front": torch.rand(3, 16, 16),
+    }
+
+    image_batches, image_masks = policy._collect_image_batches(batch)
+
+    assert len(image_batches) == 1
+    assert len(image_batches[0]) == policy.config.max_views
+    assert image_masks.tolist() == [[True, False]]
+
+
 def test_evo1_action_mask_accepts_chunk_size_one(monkeypatch):
     monkeypatch.setattr(modeling_evo1, "EVO1", DummyEVO1)
     config = make_config(chunk_size=1, n_action_steps=1)