Fixup move gluon.metric api docs (apache#18748)

* Fix metric API page

* Update index.rst

docs/python_docs/python/api/gluon/index.rst

@@ -97,7 +97,7 @@ Training
       Loss functions for training neural networks.
 
    .. card::
-      :title: mxnet.metric
+      :title: gluon.metric
       :link: metric/index.html
 
       Metrics to evaluate the performance of a learned model.

docs/python_docs/python/api/gluon/metric/index.rst

@@ -0,0 +1,23 @@
+.. Licensed to the Apache Software Foundation (ASF) under one
+   or more contributor license agreements.  See the NOTICE file
+   distributed with this work for additional information
+   regarding copyright ownership.  The ASF licenses this file
+   to you under the Apache License, Version 2.0 (the
+   "License"); you may not use this file except in compliance
+   with the License.  You may obtain a copy of the License at
+
+     http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing,
+   software distributed under the License is distributed on an
+   "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+   KIND, either express or implied.  See the License for the
+   specific language governing permissions and limitations
+   under the License.
+
+gluon.metric
+============
+
+.. automodule:: mxnet.gluon.metric
+    :members:
+    :autosummary: