Added serving module for Panoptic MaskRCNN (#10281)

* added `PanopticSegmentationModule`

* added tests for `PanopticSegmentationModule`

* added script to export `saved_model`

* added registry_imports

* added training script

* fixed docstrings

* pass model as an arg to serving module

official/vision/beta/projects/panoptic_maskrcnn/common/registry_imports.py

+# Copyright 2021 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.
+
+"""Imports to configure Panoptic Mask R-CNN"""
+
+# pylint: disable=unused-import
+from official.vision.beta.projects.panoptic_maskrcnn.configs import panoptic_maskrcnn
+from official.vision.beta.projects.panoptic_maskrcnn.tasks import panoptic_maskrcnn