Add roberta.decode to hub interface to decode BPE (#931)

Summary:
Fixes https://github.com/pytorch/fairseq/issues/930.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/931

Differential Revision: D16562511

Pulled By: myleott

fbshipit-source-id: c4c07e2f067326b79daa547dcb3db84aeddbd555

examples/roberta/README.md

@@ -60,6 +60,8 @@ $ tar -xzvf roberta.large.tar.gz
 >>> tokens = roberta.encode('Hello world!')
 >>> tokens
 tensor([    0, 31414,   232,   328,     2])
+>>> roberta.decode(tokens)
+'Hello world!'
 ```
 
 ##### Extract features from RoBERTa:

fairseq/models/roberta/hub_interface.py

@@ -3,6 +3,7 @@
 # This source code is licensed under the MIT license found in the
 # LICENSE file in the root directory of this source tree.
 
+import numpy as np
 import torch
 import torch.nn as nn
 import torch.nn.functional as F
@@ -38,6 +39,19 @@ class RobertaHubInterface(nn.Module):
         tokens = self.task.source_dictionary.encode_line(bpe_sentence, append_eos=False)
         return tokens.long()
 
+    def decode(self, tokens: torch.LongTensor):
+        assert tokens.dim() == 1
+        tokens = tokens.numpy()
+        if tokens[0] == self.task.source_dictionary.bos():
+            tokens = tokens[1:]  # remove <s>
+        eos_mask = (tokens == self.task.source_dictionary.eos())
+        doc_mask = eos_mask[1:] & eos_mask[:-1]
+        sentences = np.split(tokens, doc_mask.nonzero()[0] + 1)
+        sentences = [self.bpe.decode(self.task.source_dictionary.string(s)) for s in sentences]
+        if len(sentences) == 1:
+            return sentences[0]
+        return sentences
+
     def extract_features(self, tokens: torch.LongTensor, return_all_hiddens=False) -> torch.Tensor:
         if tokens.dim() == 1:
             tokens = tokens.unsqueeze(0)