Investigate writing custom c++/CUDA function

[rdaly525]

Note, 'N' refers to the number of select bits of a mux (or the number of input bits of a LUT).
'K' refers to the number of LUTs in a particular layer.

layers.py:135
The function LutLayer is a single layer of LUTs. This specifies the connectivity as well as instantiating the Luts.

layers.py:113
The function LutN creates the LUT weights and the Mux.

layers.py:66
The function MuxSTriangle does the actual Mux computation. 'I' is mux input, 'S' is mux select. For a LUT, the 'I' is the weights, and the 'S' is the LUT input.

For custom code, we need to speedup either the full LutLayer (which instantiates a LutN which instantiates a MuxSTriangle) or just the MuxSTriangle.