Fix fill_buffer method (#2971)

Summary:
* Add missing docsrtings
* Add default values

Pull Request resolved: https://github.com/pytorch/audio/pull/2971

Reviewed By: xiaohui-zhang

Differential Revision: D42425796

Pulled By: mthrok

fbshipit-source-id: a6a946875142a54424c059bbfbab1908a1564bd3

torchaudio/io/_stream_reader.py

@@ -753,9 +753,16 @@ class StreamReader:
         """
         return self._be.pop_chunks()
 
-    def fill_buffer(self, timeout: Optional[float], backoff: float) -> int:
+    def fill_buffer(self, timeout: Optional[float] = None, backoff: float = 10.0) -> int:
         """Keep processing packets until all buffers have at least one chunk
 
+        Arguments:
+            timeout (float or None, optional): See
+                :py:func:`~StreamReader.process_packet`. (Default: ``None``)
+
+            backoff (float, optional): See
+                :py:func:`~StreamReader.process_packet`. (Default: ``10.0``)
+
         Returns:
             int:
                 ``0``