[Cherry-pick] Fix backward compatibility layer in backend module

The PR pytorch#3549 re-organized the backend implementations and deprecated
the direct access to torchaudio.backend.

The change was supposed to be BC-compatible while issuing a warning
to users, but the implementation of module-level `__getattr__` was
not quite right.

See an issue pyannote/pyannote-audio#1456.

This commit fixes it so that the following imports work;

```python
from torchaudio.backend.common import AudioMetaData

from torchaudio.backend import sox_io_backend
from torchaudio.backend.sox_io_backend import save, load, info

from torchaudio.backend import no_backend
from torchaudio.backend.no_backend import save, load, info

from torchaudio.backend import soundfile_backend
from torchaudio.backend.soundfile_backend import save, load, info
```

torchaudio/__init__.py

@@ -1,5 +1,7 @@
+# Initialize extension and backend first
+from . import _extension  # noqa  # usort: skip
+from ._backend.common import AudioMetaData  # noqa  # usort: skip
 from . import (  # noqa: F401
-    _extension,
     compliance,
     datasets,
     functional,
@@ -11,7 +13,6 @@
     transforms,
     utils,
 )
-from ._backend.common import AudioMetaData  # noqa
 
 try:
     from .version import __version__, git_version  # noqa: F401

torchaudio/backend/__init__.py

@@ -3,40 +3,8 @@
 # New things should be added to `torchaudio._backend`.
 # Only things related to backward compatibility should be placed here.
 
-from .utils import _init_backend, get_audio_backend, list_audio_backends, set_audio_backend
 
+from . import common, no_backend, soundfile_backend, sox_io_backend  # noqa
+from .utils import _init_backend, get_audio_backend, list_audio_backends, set_audio_backend
 
 __all__ = ["_init_backend", "get_audio_backend", "list_audio_backends", "set_audio_backend"]
-
-
-def __getattr__(name: str):
-    if name == "common":
-        from . import _common
-
-        return _common
-
-    if name in ["no_backend", "sox_io_backend", "soundfile_backend"]:
-        import warnings
-
-        warnings.warn(
-            "Torchaudio's I/O functions now support par-call bakcend dispatch. "
-            "Importing backend implementation directly is no longer guaranteed to work. "
-            "Please use `backend` keyword with load/save/info function, instead of "
-            "calling the udnerlying implementation directly.",
-            stacklevel=2,
-        )
-
-        if name == "sox_io_backend":
-            from . import _sox_io_backend
-
-            return _sox_io_backend
-        if name == "soundfile_backend":
-            from torchaudio._backend import soundfile_backend
-
-            return soundfile_backend
-
-        if name == "no_backend":
-            from . import _no_backend
-
-            return _no_backend
-    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")

torchaudio/backend/_no_backend.py

@@ -2,6 +2,7 @@
 from typing import Callable, Optional, Tuple, Union
 
 from torch import Tensor
+from torchaudio import AudioMetaData
 
 
 def load(
@@ -20,5 +21,5 @@ def save(filepath: str, src: Tensor, sample_rate: int, precision: int = 16, chan
     raise RuntimeError("No audio I/O backend is available.")
 
 
-def info(filepath: str) -> None:
+def info(filepath: str) -> AudioMetaData:
     raise RuntimeError("No audio I/O backend is available.")

torchaudio/backend/no_backend.py

@@ -0,0 +1,14 @@
+def __getattr__(name: str):
+    import warnings
+
+    warnings.warn(
+        "Torchaudio's I/O functions now support par-call bakcend dispatch. "
+        "Importing backend implementation directly is no longer guaranteed to work. "
+        "Please use `backend` keyword with load/save/info function, instead of "
+        "calling the udnerlying implementation directly.",
+        stacklevel=2,
+    )
+
+    from . import _no_backend
+
+    return getattr(_no_backend, name)

torchaudio/backend/soundfile_backend.py

@@ -0,0 +1,14 @@
+def __getattr__(name: str):
+    import warnings
+
+    warnings.warn(
+        "Torchaudio's I/O functions now support par-call bakcend dispatch. "
+        "Importing backend implementation directly is no longer guaranteed to work. "
+        "Please use `backend` keyword with load/save/info function, instead of "
+        "calling the udnerlying implementation directly.",
+        stacklevel=2,
+    )
+
+    from torchaudio._backend import soundfile_backend
+
+    return getattr(soundfile_backend, name)

torchaudio/backend/sox_io_backend.py

@@ -0,0 +1,14 @@
+def __getattr__(name: str):
+    import warnings
+
+    warnings.warn(
+        "Torchaudio's I/O functions now support par-call bakcend dispatch. "
+        "Importing backend implementation directly is no longer guaranteed to work. "
+        "Please use `backend` keyword with load/save/info function, instead of "
+        "calling the udnerlying implementation directly.",
+        stacklevel=2,
+    )
+
+    from . import _sox_io_backend
+
+    return getattr(_sox_io_backend, name)