Tiny optimization to AsyncDagNet: wait on fewer events
Tiny optimization to AsyncDagNet: wait on fewer events

Summary:
Just noticed while reading the code.

We can wait only to tails of the dag, not every execution chain node.

Reviewed By: akyrola

Differential Revision: D5861078

fbshipit-source-id: f4f6296fed1ccc96b1ab99b4272b82c8bf764ca9

Yangqing/caffe2,sf-wind/caffe2,sf-wind/caffe2,xzturn/caffe2,Yangqing/caffe2,Yangqing/caffe2,sf-wind/caffe2,xzturn/caffe2,xzturn/caffe2,davinwang/caffe2,pietern/caffe2,pietern/caffe2,pietern/caffe2,pietern/caffe2,davinwang/caffe2,xzturn/caffe2,sf-wind/caffe2,davinwang/caffe2,Yangqing/caffe2,davinwang/caffe2,caffe2/caffe2,pietern/caffe2,Yangqing/caffe2,davinwang/caffe2,sf-wind/caffe2,xzturn/caffe2