fix underscore opt (#2938)
fix underscore opt (#2938)

Co-authored-by: Jing Xu <41e29e28c21dcdb91d68530af06980de0ebd1fa4@devfair0173.h2.fair>
facebookresearch/ParlAI,facebookresearch/ParlAI,facebookresearch/ParlAI,facebookresearch/ParlAI,facebookresearch/ParlAI