C++

Add multm_prev_ layer and enhance gemm() function for PLANE_WISE oper… #79

Job	Run time
ubuntu-20_04-gcc-7	26m 28s
ubuntu-latest-clang-default-avx	29m 42s
ubuntu-latest-ffmpeg7	1m 21s
ubuntu-latest-gcc-11-blas	26m 49s
ubuntu-latest-gcc-default-cmake-3-8-ffmpeg5	26m 2s
windows-2019	14m 53s
windows-latest	25m 57s
	2h 31m 12s

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