[Fix]: fix RuntimeError of SyncBuffersHook (#309)

* fix RuntimeError of SyncBuffersHook

* add UT

mmengine/dist/dist.py

@@ -94,7 +94,11 @@ def all_reduce(data: Tensor,
         # it with 'sum' operation.
         if op.lower() == 'mean':
             torch_dist.all_reduce(data_on_device, _get_reduce_op('sum'), group)
-            data_on_device.div_(world_size)  # type: ignore
+
+            # When the type of `data_on_device` is int64,
+            # `data_on_device.div_(world_size)` will  appear RuntimeError:
+            # result type Float can't be cast to  the desired output type Long.
+            data_on_device = data_on_device / world_size  # type: ignore
         else:
             torch_dist.all_reduce(data_on_device, _get_reduce_op(op), group)
 

tests/test_dist/test_dist.py

@@ -132,8 +132,9 @@ class TestDistWithGLOOBackend(MultiProcessTestCase):
 
     def test_all_reduce(self):
         self._init_dist_env(self.rank, self.world_size)
-        for tensor_type, reduce_op in zip([torch.int64, torch.float32],
-                                          ['sum', 'mean']):
+        tensor_types = [torch.int64, torch.float32, torch.int64]
+        reduce_ops = ['sum', 'mean', 'mean']
+        for tensor_type, reduce_op in zip(tensor_types, reduce_ops):
             if dist.get_rank() == 0:
                 data = torch.tensor([1, 2], dtype=tensor_type)
             else: