replaced handcoded sigmoid with backend function
replaced handcoded sigmoid with backend function

NervanaSystems/neon,NervanaSystems/neon,Jokeren/neon,NervanaSystems/neon,Jokeren/neon,coufon/neon-distributed,dongjoon-hyun/neon,dongjoon-hyun/neon,Jokeren/neon,NervanaSystems/neon,Jokeren/neon,coufon/neon-distributed,dongjoon-hyun/neon,coufon/neon-distributed