`intial_prompt` support for automatic-speech-recognition (whisper) pipeline
Author: Biswajit2902
Pull request: yes

# What does this PR do?

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Fixes # (feature)
- `initial_prompt` support for whisper Pipeline (automatic-speech-recognition) 

## Before submitting
- [ ] Added initial_prompt as an option for whisper model
- [ ] To handle initial prompt `processor` considered as optional parameter
- [ ] Current implementation supports only Torch version of decoding.
- [ ] how to use initial prompt; 

``` python
from transformers import AutoModelForSpeechSeq2Seq, AutoProcessor, pipeline
from datasets import load_dataset

model_id = "openai/whisper-small"
model = AutoModelForSpeechSeq2Seq.from_pretrained(
    model_id, torch_dtype=torch_dtype, low_cpu_mem_usage=True, use_safetensors=True
)
model.to(device)

processor = AutoProcessor.from_pretrained(model_id)

pipe = pipeline(
    "automatic-speech-recognition",
    model=model,
    tokenizer=processor.tokenizer,
    feature_extractor=processor.feature_extractor,
    max_new_tokens=128,
    chunk_length_s=15,
    batch_size=16,
    torch_dtype=torch_dtype,
    device=device,
    processor=processor
)

dataset = load_dataset("distil-whisper/librispeech_long", "clean", split="validation")
sample = dataset[0]["audio"]

# including timestamp
print(pipe(audio, initial_prompt = "Biswajit, Whisper", return_timestamps=True))

# without timestamp
print(pipe(audio, initial_prompt = "Biswajit, Whisper"))
```

## Who can review?

Anyone in the community is free to review the PR once the tests have passed. @sanchit-gandhi , @Narsil, Can anyone help to take this PR forward please. Let me know, if anything is needed.

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Models:

- text models: @ArthurZucker and @younesbelkada
- vision models: @amyeroberts
- speech models: @sanchit-gandhi
- graph models: @clefourrier

Library:

- flax: @sanchit-gandhi
- generate: @gante
- pipelines: @Narsil
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- tokenizers: @ArthurZucker
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Integrations:

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- ray/raytune: @richardliaw, @amogkam
- Big Model Inference: @SunMarc
- quantization (bitsandbytes, autogpt): @SunMarc and @younesbelkada

Documentation: @stevhliu and @MKhalusova

HF projects:

- accelerate: [different repo](https://github.com/huggingface/accelerate)
- datasets: [different repo](https://github.com/huggingface/datasets)
- diffusers: [different repo](https://github.com/huggingface/diffusers)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

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