BLD: preprare for 0.5.1 release (#481)

Author: facaiy

tensorflow_addons/version.py

@@ -20,7 +20,7 @@
 # We follow Semantic Versioning (https://semver.org/)
 _MAJOR_VERSION = '0'
 _MINOR_VERSION = '5'
-_PATCH_VERSION = '0'
+_PATCH_VERSION = '1'
 
 # When building releases, we can update this value on the release branch to
 # reflect the current release candidate ('rc0', 'rc1') or, finally, the official