Update Whisper export with beam search (#19322)
### Description
This PR updates the Whisper export with beam search by adding the
following.
- Fixes a bug when running `DecoderMaskedMultiHeadAttention` in the
Whisper with beam search model
- Sets the default PyTorch attention implementation to `eager` to allow
existing attention fusions to continue working
- Re-uses the cache directory when loading the PyTorch model to reduce
memory used on disk
- Adds `--disable_auto_mixed_precision` to the example FP16 export
command
### Motivation and Context
- [This PR](https://github.com/microsoft/onnxruntime/pull/19112) added
the `is_unidirectional` parameter to `CheckInputs`, but it was not
provided when checking the inputs in `DecoderMaskedMultiHeadAttention`.
- [This PR](https://github.com/microsoft/onnxruntime/pull/19200)
explains the reasoning behind why `eager` is used to load the
`WhisperAttention` class.
- By re-using the cache directory for loading the PyTorch model, only
one copy of the PyTorch model is saved on disk instead of two copies.
- By providing this flag, there will be less Cast nodes in the Whisper
with beam search model to switch between FP16 and FP32 precision.
