C++

Add dependency-free ONNX export for DNNs #1602

Job	Run time
ubuntu-22-04-gcc-default-cmake-3-17-ffmpeg5	26m 53s
ubuntu-latest-clang-default-avx	15m 51s
windows-latest	29m 11s
ubuntu-latest-gcc-11-blas	15m 37s
ubuntu-22-04-ffmpeg711	1m 23s
ubuntu-22-04-ffmpeg701	1m 32s
	1h 30m 27s

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