Add batched_mul support for oneAPI

[AntonOresten]

Like #614, this PR adds partial support for a previously unsupported backend. It was just a matter of hooking up the bindings, using that PR as template. I think NNlib.batched_mul is particularly useful, and it was one of my only hiccups with trying oneAPI.jl, as this function is used extensively in the ecosystem, so even partial oneAPI support would be useful.

PR Checklist

• Tests are added
• Documentation note about partial oneAPI support

[AntonOresten]

I played around with oneAPI further, and it seems it's generally not very stable.

[CarloLucibello]

This looks good. Just add the buildkite pipeline for testing. It should have the option:

        agents:
          queue: "juliagpu"
          intel: "*"