Fix _length_per_key_from_stride_per_key empty cat

Summary: torch.cat fails on empty list, guarding this case.

Reviewed By: zainhuda

Differential Revision: D54305327

fbshipit-source-id: 82877e4f307631eed816a60b35e8b1ca52104b32

Author: Ivan Kobzarev

torchrec/sparse/jagged_tensor.py

@@ -728,9 +728,13 @@ def _length_per_key_from_stride_per_key(
             1, stride_per_key_offsets, lengths
         ).tolist()
     else:
-        return torch.cat(
-            [torch.sum(chunk).view(1) for chunk in torch.split(lengths, stride_per_key)]
-        ).tolist()
+        tensor_list: List[torch.Tensor] = [
+            torch.sum(chunk).view(1) for chunk in torch.split(lengths, stride_per_key)
+        ]
+        if len(tensor_list) == 0:
+            return []
+
+        return torch.cat(tensor_list).tolist()
 
 
 def _maybe_compute_length_per_key(