Indexing fix for gpt_bigcode
The `past_key_values` is a list of tensors in gpt_bigcode, and not a list of lists like with most models, so we don't want to index twice. The code works either way but it's safer to avoid the unnecessary indexing. I also fixed the associated type hint.
@younesbelkada
_The documentation is not available anymore as the PR was closed or merged._