Do not encode supplied initial_prompt if it is not a string

[jzyeezy]

The BatchedInferencePipeline#transcribe method technically supports passing an already encoded tokenized list as the initial_prompt. The code path should only try to encode the provided prompt if it's a string.

This fixes the bug where faster_whisper will fail due to: TypeError: TextInputSequence must be str when passing a List of integers as the initial_prompt.

[MahmoudAshraf97]

Thanks for your contribution, You still need to handle the case where options.initial_prompt is None

[MahmoudAshraf97]

Also, even if the typing hints support, Why would you use a tokenized string instead of a string? note that different whisper models use different tokenizers so the tokenized string is not generalizable
I prefer if we change the typing hint to just accept str instead

[jzyeezy]

Why would you use a tokenized string instead of a string?

@MahmoudAshraf97 gotcha. I only discovered this bug while experimenting with the parameter to observe if there's any upside in providing the prompt in this format. I'm happy to update the signature of this parameter to accept only a string.