update model_split_percents
# What does this PR do?

#21883 changed `WhisperModelTest`'s `model_split_percents ` from `[0.5, 0.7, 0.9]` to `= [0.8, 0.9]` to make `test_model_parallelism` work. But it fails `test_disk_offload` which uses `self.model_split_percents[0]`.

This PR adds back `0.5`. With `model_split_percents = [0.5, 0.8, 0.9]`, the relevant Whisper tests using `model_split_percents` all pass.


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