Fix token_id values for whisper export (#15362)
### Description
The current ONNX export of Whisper utilizes hard-coded values for
token_ids when configuring the BeamSearch node. This PR removes these
literals and instead takes these values straight from the WhisperConfig.
### Motivation and Context
Hard-coding these values can cause some parity issues when comparing to
default PyTorch behavior - this change to take from WhisperConfig
resolves these.
Co-authored-by: Peter McAughan <petermca@microsoft.com>
