[Performance] Faster params and buffer registration in TensorDictPara…

…ms (#569)

tensordict/nn/params.py

@@ -317,18 +317,21 @@ def _apply_get_post_hook(self, val):
     def _reset_params(self):
         parameters = self._param_td
         param_keys = []
+        params = []
         buffer_keys = []
+        buffers = []
         for key, value in parameters.items(True, True):
+            # flatten key
+            if isinstance(key, tuple):
+                key = "_".join(key)
             if isinstance(value, nn.Parameter):
                 param_keys.append(key)
+                params.append(value)
             else:
                 buffer_keys.append(key)
-        self.__dict__["_parameters"] = (
-            parameters.select(*param_keys).flatten_keys("_").to_dict()
-        )
-        self.__dict__["_buffers"] = (
-            parameters.select(*buffer_keys).flatten_keys("_").to_dict()
-        )
+                buffers.append(value)
+        self.__dict__["_parameters"] = dict(zip(param_keys, params))
+        self.__dict__["_buffers"] = dict(zip(buffer_keys, buffers))
 
     @classmethod
     def __torch_function__(