Update PR labeling workflow (#2160)

Summary:
update the labeling reminder to be triggered when PRs are closed rather than merged, because of the transition of merging through fbcode

Pull Request resolved: https://github.com/pytorch/audio/pull/2160

Reviewed By: nateanl

Differential Revision: D33642490

Pulled By: carolineechen

fbshipit-source-id: bb39c66653782694d967303065d40386689789a8

.github/process_commit.py

@@ -66,6 +66,7 @@ if __name__ == "__main__":
     commit_hash = sys.argv[1]
 
     merger, pr_number = get_pr_merger_and_number(commit_hash)
+    if pr_number:
         labels = get_labels(pr_number)
         is_properly_labeled = bool(PRIMARY_LABELS.intersection(labels) and SECONDARY_LABELS.intersection(labels))
 

.github/workflows/pr-labels.yml

 name: pr-labels
 
 on:
-  push:
-    branches:
-      - main
+  pull_request:
+    types:
+      - closed
 
 jobs:
   is-properly-labeled: