GM error for unbatched inputs

Summary: Error when sending an unbatched FrameData through GM.

Reviewed By: shapovalov

Differential Revision: D38036286

fbshipit-source-id: b8d280c61fbbefdc112c57ccd630ab3ccce7b44e

pytorch3d/implicitron/models/generic_model.py

@@ -765,6 +765,17 @@ class GenericModel(ImplicitronModelBase, torch.nn.Module):  # pyre-ignore: 13
         Returns:
             Modified image_rgb, fg_mask, depth_map
         """
+        if image_rgb is not None and image_rgb.ndim == 3:
+            # The FrameData object is used for both frames and batches of frames,
+            # and a user might get this error if those were confused.
+            # Perhaps a user has a FrameData `fd` representing a single frame and
+            # wrote something like `model(**fd)` instead of
+            # `model(**fd.collate([fd]))`.
+            raise ValueError(
+                "Model received unbatched inputs. "
+                + "Perhaps they came from a FrameData which had not been collated."
+            )
+
         fg_mask = fg_probability
         if fg_mask is not None and self.mask_threshold > 0.0:
             # threshold masks