[fix] multi-node backward slowdown (#6134)

* remove redundant memcpy during backward

* get back record_stream

colossalai/zero/low_level/bookkeeping/bucket_store.py

@@ -78,13 +78,13 @@ class BucketStore(BaseStore):
         }
         """
         for param, padding_size in zip(self._param_list, self._padding_size):
-            grad = param.grad.clone().detach().flatten()
+            grad = param.grad.detach().flatten()
             if padding_size > 0:
                 with torch.no_grad():
                     grad = torch.nn.functional.pad(grad.view(-1), [0, padding_size])
             grad_list = grad.split(grad.numel() // self._world_size)
             for rank in range(self._world_size):
-                grad_current_rank = grad_list[rank].clone().detach()
+                grad_current_rank = grad_list[rank].detach()
                 self.grad_to_param_mapping[id(grad_current_rank)] = id(param)
                 self._grad_in_bucket[rank].append(grad_current_rank)
             param.grad = None
@@ -110,7 +110,7 @@ class BucketStore(BaseStore):
 
         flat_grad = []
         for grad_list in self._grad_in_bucket.values():
-            flat_grad.append(_flatten_dense_tensors(grad_list))
+            flat_grad.extend(grad_list)
         flat_grad = _flatten_dense_tensors(flat_grad)
         return flat_grad