ci: add missing labeler job (Lightning-AI#2038)

.github/workflows/labeler.yml

@@ -0,0 +1,18 @@
+name: "Pull Request Labeler"
+on: [pull_request_target]
+
+jobs:
+  triage-prs:
+    permissions:
+      contents: read
+      pull-requests: write
+    runs-on: ubuntu-latest
+    steps:
+    # Uploads repository content to the runner
+    - uses: actions/checkout@v3
+    - uses: actions/labeler@v4
+      with:
+        # The path to the label configuration file.
+        configuration-path: .github/labeler.yml
+        # Whether removing labels when matching files are reverted or no longer changed by the PR
+        sync-labels: true