[Fbsync] Synchronize extension initialization mechanism with fbcode (#1682)

ref: D29989551

torchaudio/extension/__init__.py

@@ -2,6 +2,11 @@ from .extension import (
     _init_extension,
 )
 
+try:
+    from . import fb  # noqa
+except Exception:
+    pass
+
 _init_extension()
 
 del _init_extension