Update eigen_backward_spatial_convolutions.h
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For the most up to date status, view the checks section at the bottom of the pull request. This code doesn't actually compile with the latest version of tensorflow - there's some kind of type mismatch that causes the convolution kernels to fail to build. If it doesn't even build, not really point in the pull request being active. It was more of an idea on how to speed up the "meat" of the implementation.
Replaced backward spatial convolution with faster and a bit more streamlined implementation based on TF 1.13.1, with some minor changes. Really shines at larger input/output Tensor sizes (128x128 and larger, batch agnostic) with at least 8 input and 8 output channels. It is upto 30% faster than the current method with 64 input/output channels, or 25% faster with 32 in_ch/out_ch .
It is sadly 2x slower on small number of channels (4 in_ch/out_ch and smaller) on my machine. Tested on Clang 16.
