Well, you're basically correct that the ML GPU libraries basically all build on CUDA with non-Nvidia being a second class citizen, but then stuff like OpenGL, Vulkan, DirectX and Metal are more general "os-level" or low level GPU graphics and compute abstractions. This, and the popular rust project `wgpu` which is the base for Firefox's WebGPU implementation abstract over these lower-level abstractions that may or may not be available such that the same, often a bit simpler code is usable accross multiple types of devices. Basically the opposite of needing CUDA.