Supports Whisper prompt and prefix

[ocavue]

Feature request

Whisper has two options prompt and prefix defined in DecodingOptions (source code). These two options will be inserted as part of the initial_tokens (source code). It would be good to support them in Transformers.js

Motivation

You can find a detailed description of these two options here: openai/whisper#117 (comment). In short, they can improve the transaction quality for long speak (i.e. larger than 30 seconds).

Your contribution

The Python library transformers doesn’t appear to include these two options either. I’m unsure whether transformers and transformers.js need to maintain alignment. If these two libraries don’t need to share the same options, I can attempt to add prompt and prefix support to transformers.js.