Add automatic tpu label to mergify.yml (#15560)

.github/mergify.yml

@@ -88,6 +88,17 @@ pull_request_rules:
       add:
         - v1
 
+- name: label-tpu
+  description: Automatically apply tpu label
+  conditions:
+    - or:
+      - files~=tpu
+      - files~=pallas
+  actions:
+    label:
+      add:
+        - tpu
+
 - name: ping author on conflicts and add 'needs-rebase' label
   conditions:
       - conflict