add optimized element-wise cpu function with proper broadcasting
add optimized element-wise cpu function with proper broadcasting

emjotde/amunmt,emjotde/Marian,amunmt/marian,marian-nmt/marian-train,marian-nmt/marian-train,marian-nmt/marian-train,emjotde/amunmt,amunmt/marian,emjotde/amunmt,emjotde/amunn,emjotde/amunn,marian-nmt/marian-train,amunmt/marian,emjotde/amunn,emjotde/amunmt,emjotde/amunn,marian-nmt/marian-train,emjotde/Marian