add nll loss

fastNLP/core/loss.py

@@ -37,5 +37,7 @@ class Loss(object):
         """
         if loss_name == "cross_entropy":
             return torch.nn.CrossEntropyLoss()
+        elif loss_name == 'nll':
+            return torch.nn.NLLLoss()
         else:
             raise NotImplementedError