parallelize flatten transform
parallelize flatten transform

Reviewed By: alunyov

Differential Revision: D21904518

fbshipit-source-id: 366e4bddc9d505fd32a50c1f1ad66200ece1029e

josephsavona/relay,voideanvalue/relay,wincent/relay,xuorig/relay,facebook/relay,wincent/relay,kassens/relay,voideanvalue/relay,voideanvalue/relay,xuorig/relay,atxwebs/relay,voideanvalue/relay,kassens/relay,voideanvalue/relay,xuorig/relay,wincent/relay,yungsters/relay,josephsavona/relay,atxwebs/relay,voideanvalue/relay,atxwebs/relay,xuorig/relay,wincent/relay,wincent/relay,facebook/relay,josephsavona/relay,kassens/relay,yungsters/relay,facebook/relay,kassens/relay,wincent/relay,kassens/relay,facebook/relay,xuorig/relay,facebook/relay,xuorig/relay,facebook/relay,yungsters/relay,kassens/relay,atxwebs/relay