Add support for AbortController on downloading model

[flatsiedatsie]

Sometimes a model will start downloading, but then you realize it's the wrong one, or it's going to take too long, etc.

I'd like to offer users a "cancel download" button. But how would I implement it cleanly? For example, so that already downloaded chunks are also removed, and Wllama ends up in an 'nothing loaded yet' state.

[ngxson]

Probably it can be archived using fetch API AbortController