docs(onnx): add a note when the onnxruntime_gpu dependency is missing.

[dongfeng3692]

No description provided.

[tzolov]

Thanks you @dongfeng3692
Valuable insight. I was wondering if we should add the onnxruntime_gpu dependency by default, but it appears that it is not supported on all platforms yet.

[dongfeng3692]

@tzolov
I do not recommend adding onnxruntime_gpu as a default dependency. Different versions of the dependency support different CUDA versions, and the onnxruntime_gpu package is large, for example, version 1.18.0 is 357 MB. Such a large dependency could cause inconvenience to users who do not use GPUs.

[tzolov]

Thanks for the elaboration @dongfeng3692
Agree that it will be too heavy to add this deps