Fixed confusing order of args in generate() docstring
Author: RafaelWO
Pull request: yes

# What does this PR do?

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This PR addresses a (IMO) confusing parameter description in the docstring of `generate()`. Specifically, it is about the parameter `prefix_allowed_tokens_fn` which has  to be of type `Callable[[int, torch.Tensor], List[int]]`. Since the description says _"This function takes 2 arguments `inputs_ids` and the batch ID `batch_id`"_ I created a function
```Python
def restrict_vocab(input_ids, batch_id):   # incorrect!
    # logic
```
But then I realised that the order of the parameters is wrong (the type hint would indicate that `batch_id` comes first though):
```Python
def restrict_vocab(batch_id, input_ids):   # correct :)
    # logic
```

Therefore, I fixed the order of the parameters in the description of `prefix_allowed_tokens_fn`, i.e. exchanged `inputs_ids` with `batch_id`. Now it should be easier to read and understand.


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