Implement `torch.bucketize` and `torch.unfold`

[EricLBuehler]

Hello @mokeyish,

Thanks for your excellent work here.

We are implementing Idefics 2 in mistral.rs, but the Hugging Face implementation we are basing it on uses torch.bucketize and torch.unfold. Given your experience here implementing several extensions to Candle, would you be able to implement those functions? Thank you in advance!

[mokeyish]

@EricLBuehler It would be better if you are willing to add them to this library. I created this library because I wanted to migrate the model to candle, and candle lacked these functions. But I haven't done it recently because I am busy with other things.