stage3.py: do not scale if gradient_predivide_factor is 1.0 (#3630)

this change also aligns with the logic before reduce_scatter_coalesced
Co-authored-by: NOlatunji Ruwase <olruwase@microsoft.com>

deepspeed/runtime/zero/stage3.py

@@ -1122,7 +1122,8 @@ class DeepSpeedZeroOptimizer_Stage3(ZeROOptimizer):
 
         grad_partitions_for_rank = reduce_scatter_coalesced(full_grads_for_rank, self.dp_process_group)
 
-        if self.postscale_gradients and self.gradient_predivide_factor != dist.get_world_size(self.dp_process_group):
+        if self.postscale_gradients and self.gradient_predivide_factor != 1.0 and self.gradient_predivide_factor != dist.get_world_size(
+                self.dp_process_group):
             grad_partitions_for_rank = [g.mul(self.gradient_predivide_factor) for g in grad_partitions_for_rank]
 
         if self.communication_data_type != self.dtype: