[WIP v2 - deprecated] Unlikelihood token loss#2011

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funboarder13920 wants to merge 4 commits intoOpenNMT:masterfrom funboarder13920:unlikelihood_loss_and_ppl

+400-134

Commits on Feb 19, 2021

implement unlikelihood token loss and fix ppl to always be the ppl
Valentin Berkes
committed
address PR comments + commit missing test file
Valentin Berkes
committed
mutually exclusive label_smoothing and unlikelihood_coeff
Valentin Berkes
committed
merge LossComputeBase and CommonLossComputeBase
Valentin Berkes
committed