Better error message when dataset root set wrongly in JsonIndexDatasetMapProviderV2

Summary: <see title>

Reviewed By: bottler

Differential Revision: D38314727

fbshipit-source-id: 7178b816a22b06af938a35c5f7bb88404fb1b1c4

pytorch3d/implicitron/dataset/json_index_dataset_map_provider_v2.py

@@ -177,6 +177,18 @@ class JsonIndexDatasetMapProviderV2(DatasetMapProviderBase):  # pyre-ignore [13]
 
         path_manager = self.path_manager_factory.get()
 
+        if path_manager is not None:
+            path_managed_frame_file = path_manager.get_local_path(frame_file)
+            if not os.path.isfile(path_managed_frame_file):
+                # The frame_file does not exist.
+                # Most probably the user has not specified the root folder.
+                raise ValueError(
+                    f"Looking for frame annotations in {path_managed_frame_file}."
+                    + " Please specify a correct dataset_root folder."
+                    + " Note: By default the root folder is taken from the"
+                    + " CO3DV2_DATASET_ROOT environment variable."
+                )
+
         # setup the common dataset arguments
         common_dataset_kwargs = getattr(self, f"dataset_{self.dataset_class_type}_args")
         common_dataset_kwargs = {