Create separate worker version file

----Release Notes----
[]
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=122492763

Author: charlesccychen

google/cloud/dataflow/worker/version.py

@@ -0,0 +1,17 @@
+# Copyright 2016 Google Inc. 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.
+
+"""Dataflow Python worker version information."""
+
+__version__ = '0.2.4'