[CI] Only run integration test when PR's base branch is main (#1331)

As titled, to save some H100 resource and avoid long waiting, only run
integration test when PR's base branch is main.

No need to run H100 tests on PRs like #1330

Author: wwwjn

.github/workflows/integration_test_8gpu_h100.yaml

@@ -6,6 +6,7 @@ on:
     paths-ignore:
       - 'torchtitan/experiments/**'
   pull_request:
+    branches: [ main ]
     paths-ignore:
       - 'torchtitan/experiments/**'
   schedule: