Update version.py

Author: mihirparadkar

tensorflow_gnn/version.py

@@ -41,4 +41,4 @@
 # IMPORANT: Right after branching rX.Y, bump the main branch to X.(Y+1).0.dev1.
 # (Submit a change to the Source of Truth, get it out on the main branch asap.)
 
-__version__ = "0.4.0rc1"
+__version__ = "0.4.0rc2"