AD Cleanup: Unify how the different transcribers work, and cleanup their dispatch logic

[saipraveenb25]

At the moment we have 4 different transcribers that are implemented rather weirdly.

IRBackwardDiffTranscriber generates the derivative function in transcribeFuncHeader rather than just generating the header, while the other 3 versions all generate only the header.
Further, calling transcriberObject.transcribe does not actually generate the function, only the header, and follow-up tasks are added to a global context that the caller has to follow-up by calling transcribeFunc explciitly on each function body.

It may be better to unify this logic with a single point of dispatch, with follow-up transcription requests returned as the result of the .transcribe() call.

Additionally, the word 'transcribe' might not be the best description (since it usually means audio-to-text). Calling them 'generators', 'transformers' or 'translators' might be more apt (thought this isn't really a pressing issue)