Add metrics module (#247)

* add Metrics subpackage

Author: AakashKumarNain

tensorflow_addons/BUILD

@@ -13,6 +13,7 @@ py_library(
         "//tensorflow_addons/image",
         "//tensorflow_addons/layers",
         "//tensorflow_addons/losses",
+        "//tensorflow_addons/metrics",
         "//tensorflow_addons/optimizers",
         "//tensorflow_addons/rnn",
         "//tensorflow_addons/seq2seq",

tensorflow_addons/__init__.py

@@ -75,6 +75,7 @@ def _ensure_tf_install():
 from tensorflow_addons import image
 from tensorflow_addons import layers
 from tensorflow_addons import losses
+from tensorflow_addons import metrics
 from tensorflow_addons import optimizers
 from tensorflow_addons import rnn
 from tensorflow_addons import seq2seq

tensorflow_addons/metrics/BUILD

@@ -0,0 +1,14 @@
+licenses(["notice"])  # Apache 2.0
+
+package(default_visibility = ["//visibility:public"])
+
+py_library(
+    name = "metrics",
+    srcs = [
+        "__init__.py",
+    ],
+    srcs_version = "PY2AND3",
+    deps = [
+        "//tensorflow_addons/utils",
+    ],
+)

tensorflow_addons/metrics/README.md

@@ -0,0 +1,32 @@
+# Addons - Metrics
+
+## Maintainers
+| Submodule  | Maintainers  | Contact Info   |
+|:---------- |:------------- |:--------------|
+|  |  |  |
+
+## Contents
+| Submodule | Activation  | Reference                               |
+|:----------------------- |:-------------------|:---------------|
+|  |  |            |
+
+
+## Contribution Guidelines
+#### Standard API
+In order to conform with the current API standard, all metrics
+must:
+ * Inherit from `tf.metrics.Metric`.
+ * [Register as a keras global object](https://github.com/tensorflow/addons/blob/master/tensorflow_addons/utils/python/keras_utils.py)
+  so it can be serialized properly.
+ * Add the addon to the `py_library` in this sub-package's BUILD file.
+
+#### Testing Requirements
+ * Simple unittests that demonstrate the metric is behaving as expected.
+ * When applicable, run all unittests with TensorFlow's
+   `@run_in_graph_and_eager_modes` (for test method)
+   or `run_all_in_graph_and_eager_modes` (for TestCase subclass)
+   decorator.
+ * Add a `py_test` to this sub-package's BUILD file.
+
+#### Documentation Requirements
+ * Update the table of contents in this sub-package's README.

tensorflow_addons/metrics/__init__.py

@@ -0,0 +1,19 @@
+# Copyright 2019 The TensorFlow Authors. All Rights Reserved.
+#
+# Licensed 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.
+# ==============================================================================
+"""A module containing metrics that conform to Keras API."""
+
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function