Set the version for the TFP 0.12.1 release.

Author: jburnim

tensorflow_probability/python/version.py

@@ -17,7 +17,7 @@
 # We follow Semantic Versioning (https://semver.org/)
 _MAJOR_VERSION = '0'
 _MINOR_VERSION = '12'
-_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