Simplify extension initialization (#1649)

torchaudio/extension/extension.py

 import warnings
-import importlib
 
 import torch
 from torchaudio._internal import module_utils as _mod_utils
 
 
 def _init_extension():
-    ext = 'torchaudio._torchaudio'
-    if _mod_utils.is_module_available(ext):
-        _init_script_module(ext)
-        import torchaudio._torchaudio  # noqa
+    if _mod_utils.is_module_available('torchaudio._torchaudio'):
+        # Note this import has two purposes
+        # 1. to extract the path of the extension module so that
+        #    we can initialize the script module with the path.
+        # 2. so that torchaudio._torchaudio is accessible in other modules.
+        #    Look at sox_io_backend which uses `torchaudio._torchaudio.XXX`,
+        #    assuming that the module `_torchaudio` is accessible.
+        import torchaudio._torchaudio
+        _init_script_module(torchaudio._torchaudio.__file__)
     else:
         warnings.warn('torchaudio C++ extension is not available.')
 
 
-def _init_script_module(module):
-    path = importlib.util.find_spec(module).origin
+def _init_script_module(path):
     torch.classes.load_library(path)
     torch.ops.load_library(path)