Perplexity with `torch.exp2` instead of `torch.exp`

[malteos]

Hi,

thanks for your work on this project!

I was surprised the see that your perplexity implementation uses the base two exponential.

See https://github.com/huggingface/evaluate/blob/main/metrics/perplexity/perplexity.py#L183

Is this indented or a bug?