don't recompute small blob in attention
don't recompute small blob in attention

Summary: decoder_hidden_encoder_outputs_sum_tmp is tiny after D5010109, no need to recompute it.

Reviewed By: akyrola

Differential Revision: D5014335

fbshipit-source-id: cc9e8f91372889d10bd99c79366018cb3943a435

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