[ci/build] Add placeholder command for custom models test (#9262)

.buildkite/test-pipeline.yaml

@@ -347,10 +347,11 @@ steps:
     - pytest -v -s models/encoder_decoder/language
     - pytest -v -s models/encoder_decoder/vision_language
 
+# This test is used only in PR development phase to test individual models and should never run on main
 - label: Custom Models Test
-  #mirror_hardwares: [amd]
   optional: true
   commands:
+    - echo 'Testing custom models...'
     # PR authors can temporarily add commands below to test individual models
     # e.g. pytest -v -s models/encoder_decoder/vision_language/test_mllama.py
     # *To avoid merge conflicts, remember to REMOVE (not just comment out) them before merging the PR*