Whisper: fix prompted max length
# What does this PR do?

Fixes #24600 

#23724 Added the ability to guide generation with Whiper through `prompt_ids`. It was increasing the generation length by the length of the prompt -- these tokens were being hardcoded, and thus "not generated".

However, in the default case, we were already setting the generation length to the maximum allowed model length (see [model config](https://huggingface.co/openai/whisper-large-v2/blob/main/config.json#L42)). This increment was forcing us to go behind the maximum length and, because the model uses a `nn.Embedding` for the position embedding, indexing exceptions started popping up on long audio inputs :D

This PR modifies the length extension to what I believe was the author's original goal: only increment the length if `max_new_tokens` is passed. By default, this argument is not set and should correspond to the "new" (=non-prompt) generated tokens.
_The documentation is not available anymore as the PR was closed or merged._<|||||>@amyeroberts @sanchit-gandhi 

After the latest changes, a warning is emitted when we cross `config.max_position_embeddings` for the first time.

For instance, if you now run 

```py
from transformers import AutoModelForCausalLM, AutoTokenizer

tokenizer = AutoTokenizer.from_pretrained("distilgpt2")
model = AutoModelForCausalLM.from_pretrained("distilgpt2").to("cuda")

inputs = tokenizer(["The quick brown"], return_tensors="pt").to("cuda")
# distilgpt2 has a maximum length of 1024
gen_out = model.generate(**inputs, do_sample=True, eos_token_id=-1, max_length=1025)
```

You'll see

```
This is a friendly reminder - the current text generation call will exceed the model's predefined maximum length (1024). Depending on the model, you may observe exceptions, performance degradation, or nothing at all.
```

(And if you set `max_length=1026`, you'll see the warning right before the exceptions. This is because we can technically generate `config.max_position_embeddings + 1` tokens even with restrictive position embeddings, although we shouldn't!)