Update `model_split_percents` for `WhisperModelTest` (#21922)

Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>

tests/models/whisper/test_modeling_whisper.py

@@ -285,7 +285,8 @@ class WhisperModelTest(ModelTesterMixin, GenerationTesterMixin, PipelineTesterMi
     test_pruning = False
     test_missing_keys = False
     # Needs higher percentages after model tester's vocab_size is changed to 200 (PR #21222)
-    model_split_percents = [0.8, 0.9]
+    # `0.5` is for `test_disk_offload` (which also works for `test_model_parallelism`)
+    model_split_percents = [0.5, 0.8, 0.9]
 
     input_name = "input_features"