[WhisperForCausalLM] Add WhisperForCausalLM for speculative decoding (#27195)
* finish
* add tests
* fix all tests
* [Assistant Decoding] Add test
* fix more
* better
* finish
* Apply suggestions from code review
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>
* finish
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Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>