hparams: extract api wrapper over summary_v2

Summary:
We’ll shortly extract the Keras callback out of `summary_v2` such that
`summary_v2` does not need to depend on TensorFlow. It will continue to
be exposed from `api`.

Part of a reorganization to match the structure described here:
<#2139 (comment)>

wchargin-branch: hparams-api-refactor

tensorboard/plugins/hparams/BUILD

@@ -98,9 +98,9 @@ py_binary(
     srcs = ["hparams_demo.py"],
     srcs_version = "PY2AND3",
     deps = [
+        ":api",
         ":protos_all_py_pb2",
         ":summary",
-        ":summary_v2",
         "//tensorboard:expect_absl_app_installed",
         "//tensorboard:expect_absl_flags_installed",
         "//tensorboard:expect_numpy_installed",
@@ -149,6 +149,30 @@ sh_test(
     ],
 )
 
+py_library(
+    name = "api",
+    srcs = ["api.py"],
+    srcs_version = "PY2AND3",
+    visibility = [
+        "//visibility:public",
+    ],
+    deps = [
+        ":summary_v2",
+    ],
+)
+
+py_test(
+    name = "api_test",
+    size = "small",
+    srcs = ["api_test.py"],
+    srcs_version = "PY2AND3",
+    deps = [
+        ":api",
+        ":summary_v2",
+        "//tensorboard:test",
+    ],
+)
+
 py_library(
     name = "summary_v2",
     srcs = ["summary_v2.py"],

tensorboard/plugins/hparams/api.py

@@ -0,0 +1,37 @@
+# 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.
+# ==============================================================================
+"""Experimental public APIs for the HParams plugin."""
+
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+
+from tensorboard.plugins.hparams import summary_v2
+
+
+Discrete = summary_v2.Discrete
+Domain = summary_v2.Domain
+HParam = summary_v2.HParam
+IntInterval = summary_v2.IntInterval
+KerasCallback = summary_v2.KerasCallback
+Metric = summary_v2.Metric
+RealInterval = summary_v2.RealInterval
+hparams_config = summary_v2.hparams_config
+
+
+del absolute_import
+del division
+del print_function
+del summary_v2

tensorboard/plugins/hparams/api_test.py

@@ -0,0 +1,35 @@
+# 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.
+# ==============================================================================
+
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+
+from tensorboard import test
+from tensorboard.plugins.hparams import api
+from tensorboard.plugins.hparams import summary_v2
+
+
+class ApiTest(test.TestCase):
+
+  def test_has_core_attributes(self):
+    self.assertIs(api.HParam, summary_v2.HParam)
+
+  def test_has_keras_dependent_attributes(self):
+    self.assertIs(api.KerasCallback, summary_v2.KerasCallback)
+
+
+if __name__ == "__main__":
+  test.main()

tensorboard/plugins/hparams/hparams_demo.py

@@ -36,7 +36,7 @@
 from six.moves import xrange  # pylint: disable=redefined-builtin
 import tensorflow as tf
 
-from tensorboard.plugins.hparams import summary_v2 as hp
+from tensorboard.plugins.hparams import api as hp
 
 
 if int(tf.__version__.split(".")[0]) < 2: