增加metric测试用例，修改metric的 setattr方法

fastNLP/core/metrics/metric.py

@@ -107,6 +107,9 @@ class Metric:
                                 f"instead of {type(value)}.")
         if isinstance(value, Element) and key not in self.elements:
             raise RuntimeError("Please use register_element() function to add Element.")
+        attrs = self.__dict__
+        if key in attrs and isinstance(value, Element):
+            raise RuntimeError(f'`{key}` has been registered as an attribute, cannot be registered as an Element!')
         object.__setattr__(self, key, value)
 
     # 当调用 __getattribute__ 没有找到时才会触发这个, 保留这个的目的只是为了防止 ide 的 warning

tests/core/metrics/test_metric.py

@@ -0,0 +1,38 @@
+import pytest
+from fastNLP import Metric
+
+
+class DemoMetric(Metric):
+
+    def __init__(self):
+        super().__init__(backend='torch')
+        self.count = 0
+        self.register_element('count', 0)
+
+    def evaluate(self):
+        self.count += 1
+        print(self.count)
+
+
+class DemoMetric1(Metric):
+
+    def __init__(self):
+        super().__init__(backend='torch')
+        self.register_element('count', 0)
+        self.count = 2
+
+    def evaluate(self):
+        self.count += 1
+        return self.count
+
+
+class TestMetric:
+
+    def test_v1(self):
+        with pytest.raises(RuntimeError):
+            dmtr = DemoMetric()
+            dmtr.evaluate()
+
+    def test_v2(self):
+        dmtr = DemoMetric1()
+        assert 3 == dmtr.evaluate()