update ignore_prefix_size in label_smoothed_cross_entropy
update ignore_prefix_size in label_smoothed_cross_entropy

Summary:
update ignore_prefix_size in label_smoothed_cross_entropy
- lprobs is always B x T x C in the current models
- lprobs.batch_first was default to `False` which contradicts the fact above

Reviewed By: sravyapopuri388

Differential Revision: D33304121

fbshipit-source-id: 9391b48c7036642d9741d254b03c46389a4fe584

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq