Revert "Update mosaic_fsdp_utils.py (#3185)" (#3187)

This reverts commit 2a262b4.

composer/trainer/mosaic_fsdp_utils.py

@@ -177,11 +177,16 @@ def _get_process_group(pg, process_group_cache=None):
         return process_group_cache[ranks]
 
     log.info(f'Instantiating custom process groups with {ranks=} on rank={dist.get_global_rank()}.')
-    process_group = distributed.new_group(ranks)
+
+    ranks_per_subgroup_list = list(set(dist.all_gather_object(ranks)))
+    (
+        current_group,
+        _subgroups,
+    ) = distributed.distributed_c10d.new_subgroups_by_enumeration(ranks_per_subgroup_list)
 
     if process_group_cache is not None:
-        process_group_cache[ranks] = process_group
-    return process_group
+        process_group_cache[ranks] = current_group
+    return current_group
 
 
 def _set_custom_fsdp_module_kwargs(module_kwargs: Dict, process_group_cache: Dict[Tuple[int], Any]) -> Dict: