Only run 'extended' test if run-extended label is set (#8579)

Author: NicolasHug

.github/workflows/tests.yml

@@ -139,6 +139,7 @@ jobs:
 
   unittests-extended:
     uses: pytorch/test-infra/.github/workflows/linux_job.yml@main
+    if: contains(github.event.pull_request.labels.*.name, 'run-extended')
     with:
       repository: pytorch/vision
       test-infra-ref: main