Fix type for lm parameter in decoder (#2273)

Summary: Pull Request resolved: https://github.com/pytorch/audio/pull/2273

Reviewed By: mthrok

Differential Revision: D34799335

Pulled By: carolineechen

fbshipit-source-id: d0eea79448efdbd84758a3f433ab9350b4c94e91

torchaudio/prototype/ctc_decoder/ctc_decoder.py

@@ -181,7 +181,7 @@ class LexiconDecoder:
 def lexicon_decoder(
     lexicon: str,
     tokens: Union[str, List[str]],
-    lm: str = None,
+    lm: Optional[str] = None,
     nbest: int = 1,
     beam_size: int = 50,
     beam_size_token: Optional[int] = None,