Custom CUDA extensions

[msaroufim]

We'd like to make it really easy for people to add support for custom CUDA extensions in ao and there's a few pieces of work we need to do to get there

• Get an example of a custom cuda extension working Custom CUDA extensions #135
• Add a tutorial for a custom cuda extension louder warning + docs for custom cuda extensions #186
• Make missing cuda toolkit error louder and more obvious louder warning + docs for custom cuda extensions #186
• Ensure cuda extensions are built using manylinux - thanks @seemethere

Follow up work in a separate issue

• Make an example without premium runners - you can build cuda extensions without a cuda machine per @malfet
• Add a useful kernel people should be using like paged attention