Fix issue #3502: "Message already contains key `dyn_batch_idx`" (#3505)
Fix issue #3502: "Message already contains key `dyn_batch_idx`" (#3505)


facebookresearch/ParlAI,facebookresearch/ParlAI,facebookresearch/ParlAI,facebookresearch/ParlAI,facebookresearch/ParlAI