fix validation callback error compute to work with RNNs
fix validation callback error compute to work with RNNs

Bam4d/neon,coufon/neon-distributed,nhynes/neon,DougFirErickson/neon,NervanaSystems/neon,nhynes/neon,dongjoon-hyun/neon,coufon/neon-distributed,NervanaSystems/neon,dongjoon-hyun/neon,Bam4d/neon,DougFirErickson/neon,NervanaSystems/neon,Jokeren/neon,dongjoon-hyun/neon,NervanaSystems/neon,nhynes/neon,Jokeren/neon,Jokeren/neon,Jokeren/neon,Bam4d/neon,DougFirErickson/neon,coufon/neon-distributed