update DataSet使得可以与之前老版本的DataSet兼容

fastNLP/core/dataset.py

@@ -406,7 +406,18 @@ class DataSet(object):
 
             else:
                 raise ValueError("data only be dict or list type.")
-        self.collater = Collater()
+        self._collater = Collater()
+
+    @property
+    def collater(self):
+        if self._collater is None:
+            self._collater = Collater()
+        return self._collater
+
+    @collater.setter
+    def collater(self, value):
+        assert isinstance(value, Collater)
+        self._collater = value
 
     def __contains__(self, item):
         return item in self.field_arrays