clarify docs around onnx packages

[ssube]

While onnx is always needed, users should usually only have one of onnxruntime, onnxruntime-gpu, or onnxruntime-directml installed. The onnxruntime package is for CPU usage, while onnxruntime-gpu and onnxruntime-directml are for the GPU and also GPU. When using the onnxruntime-directml, it also helps to install the nightly version, which can be faster.

For torch, it seems like only torch itself is really needed.

[ssube]

This is mostly done, but I need to clarify when onnxruntime needs to be installed: microsoft/onnxruntime#7748 (comment) suggests you should not have both onnxruntime and onnxruntime-gpu installed together.