I’m by no means an expert in any of this. I mainly work on video processing using the GPU. That said, I would think if any library would do load balancing between them, it would likely be the Accelerate.framework that ships with the system.
However, I do have some experience with having the same code run on the GPU and the CPU. In my work, we have tried breaking images (usually frames of video) into various sized chunks and processing on both the CPU and GPU at the same time. Our conclusion is that the overhead of using both outweighs any benefit you’d get. The GPU is so much faster than the CPU, there’s no point in involving the CPU at all. These experiments were done several years ago, so perhaps the landscape has changed since then, but that was what we found.
They're great presentations with a lot of depth in the notes. I think videos are around somewhere if you prefer that.
Two specifics I'd mention:
It seems a lot of games now use feedback between frames as a way to tolerate the latency of moving data between CPU and GPU. Eg the CPU will use GPU crunched data from the previous frame as a source for CPU crunching that optimizes what data gets passed to the GPU next.
The other is that fixed functionality is moving into shaders. Unreal 5 uses a mix of hardware rasterization and software rasterization in a shader (and path tracing now as well). There the tradeoff between the two is triangle size in pixels.
They're great. I dunno if you find 3d interesting vs video, but the section in that nanite presentation where he goes through how he arrived at the LoD clustering design is some of the smartest stuff I've ever seen any developer say, ever. Like John Carmack probably saw this and went "dang, wish I'd thought of that" levels of smart.
However, I do have some experience with having the same code run on the GPU and the CPU. In my work, we have tried breaking images (usually frames of video) into various sized chunks and processing on both the CPU and GPU at the same time. Our conclusion is that the overhead of using both outweighs any benefit you’d get. The GPU is so much faster than the CPU, there’s no point in involving the CPU at all. These experiments were done several years ago, so perhaps the landscape has changed since then, but that was what we found.