fix bugs in fp16 for dp (#38405)

python/paddle/fluid/dygraph/parallel.py

@@ -356,7 +356,8 @@ def sync_params_buffers(model,
                         src_rank=0,
                         is_model_parallel=False):
     model_vars = []
-    for _, param in model.state_dict().items():
+    params_buffers = model.parameters() + model.buffers()
+    for param in params_buffers:
         if not isinstance(param, core.VarBase):
             raise TypeError("The data type of '%s' must be Varbase" %
                             param.name)