Add 'ppl' to tensorboard (#1212)

Summary:
Originally, the 'ppl' is calculated but returned as a string, which will not be printed to the tensorboard.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/1212

Differential Revision: D18339553

Pulled By: myleott

fbshipit-source-id: 52e64d5d173bfd79836a72ee103cb25c8bb2a4c2

fairseq/utils.py

@@ -306,7 +306,7 @@ def log_softmax(x, dim, onnx_trace=False):
 
 def get_perplexity(loss):
     try:
-        return '{:.2f}'.format(math.pow(2, loss))
+        return float('{:.2f}'.format(math.pow(2, loss)))
     except OverflowError:
         return float('inf')