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Flux but its not even close to PyTorch or TF in terms of features and performance
It depends what you're doing. If you need to write your own kernels, or have small networks where the framework overhead is significant, then it's way faster than Tensorflow (unless you implement your own Tensorflow OP in C++/Cuda, but that's way more painful than just implementing it directly in Flux/Julia). It's hence quite nice for research on new architectures. Flux's autodiff also handles more language features…
I don't know exactly what 'OP' means, but there are other ways to do ML in for example C++. I have some good experience with dlib, PyTorch's libtorch is on my todo list.