Update the code owners (#1352)

Author: fegin

.github/CODEOWNERS

@@ -4,7 +4,7 @@
 # These owners will be the default owners for everything in
 # the repo. Unless a later match takes precedence,
 # they will be requested for review when someone opens a pull request.
-* @tianyu-l @fegin @wwwjn
+* @tianyu-l @fegin @wwwjn @wconstab
 
 # Exclude the experiments directory by adding a pattern without owners
 /torchtitan/experiments/