Fix `get_decoder_prompt_ids` in whisper
Author: bofenghuang
Pull request: yes

# What does this PR do?

Hi @sanchit-gandhi,

I think there is one missed line in https://github.com/huggingface/transformers/pull/20589. I've added it back in this PR.

The `forced_decoder_ids` should be something like `[[<token/position>, <token/id>], ...]`. But `prefix_tokens` only returns only token ids.

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Comments:
I found this cause I got the following error when running the code in main branch

```
File ~/transformers/src/transformers/generation/utils.py:867, in GenerationMixin._get_logits_processor(self, repetition_penalty, no_repeat_ngram_size, encoder_no_repeat_ngram_size, input_ids_seq_length, encoder_input_ids, bad_words_ids, min_length, max_length, eos_token_id, forced_bos_token_id, forced_eos_token_id, prefix_allowed_tokens_fn, num_beams, num_beam_groups, diversity_penalty, remove_invalid_values, exponential_decay_length_penalty, logits_processor, renormalize_logits, suppress_tokens, begin_suppress_tokens, forced_decoder_ids)
    865     begin_index = begin_index if (input_ids_seq_length > 1 or forced_bos_token_id is None) else begin_index + 1
    866     if forced_decoder_ids is not None:
--> 867         begin_index += forced_decoder_ids[-1][0]  # generation starts after the last token that is forced
    868     processors.append(SuppressTokensAtBeginLogitsProcessor(begin_suppress_tokens, begin_index))
    869 if forced_decoder_ids is not None:

TypeError: 'int' object is not subscriptable
```

Duplicate of https://github.com/huggingface/transformers/pull/20599

Hey @bofenghuang! Sorry about that, hoping to merge the fix ASAP

@sanchit-gandhi no problem, thanks for the quick fix!