message: Do not repack gradients if number of gradients is less than number of packs.
diff: @@ -1087,6 +1087,10 @@ class CollectiveAllReduce(CrossDeviceOps):
  # ...
  # ]
  + # No chunking if number of variables is fewer than number of packs.
  + if len(chunked_by_var) < num_packs:
  + return [chunked_by_var]
  +
  # First n-1 chunks get `chunk_size` grads, last chunk gets leftover grads.
  # This strategy can cause the last chunk to have larger size compared to the
  # first n-1 chunks. Alternatively, we can increment chunk_size by 1 to get
  