added option for no PG validation for faster init (#1161)

Co-authored-by: Naman Goyal <[email protected]>

Author: ngoyal2707

fairscale/nn/data_parallel/fully_sharded_data_parallel.py

@@ -368,6 +368,7 @@ def __init__(
         gradient_predivide_factor: Optional[float] = None,
         limit_all_gather_events: bool = False,
         limit_reduce_scatter_events: bool = False,
+        should_validate_process_group: bool = True,
     ):
         try:
             import torch._C
@@ -451,7 +452,7 @@ def __init__(
             raise ValueError(f"offload type: '{offload_config.offload_type}' requires flatten_parameters=True")
 
         # skip validation if the process group was created above
-        if process_group:
+        if process_group and should_validate_process_group:
             validate_process_group(self.compute_device, self.process_group)
 
         # enable pytorch sync_bn just in case model contains sync_bn layers.