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.

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

@@ -1,10 +1,16 @@
 # Initialize extension and backend first
-from . import (  # noqa  # usort: skip
-    _extension,
-    _backend,
+from . import _extension  # noqa  # usort: skip
+from ._backend import (  # noqa  # usort: skip
+    AudioMetaData,
+    get_audio_backend,
+    info,
+    list_audio_backends,
+    load,
+    save,
+    set_audio_backend,
 )
+
 from . import (  # noqa: F401
-    backend,  # For BC
     compliance,
     datasets,
     functional,
@@ -16,7 +22,9 @@
     transforms,
     utils,
 )
-from ._backend import AudioMetaData, get_audio_backend, info, list_audio_backends, load, save, set_audio_backend
+
+# For BC
+from . import backend  # noqa # usort: skip
 
 try:
     from .version import __version__, git_version  # noqa: F401

torchaudio/backend/__init__.py

@@ -3,35 +3,6 @@
 # New things should be added to `torchaudio._backend`.
 # Only things related to backward compatibility should be placed here.
 
+from . import common, no_backend, soundfile_backend, sox_io_backend  # noqa
 
-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}")
+__all__ = []

torchaudio/backend/no_backend.py

@@ -0,0 +1,49 @@
+from pathlib import Path
+from typing import Callable, Optional, Tuple, Union
+
+import torchaudio
+
+from torch import Tensor
+
+
+def __getattr__(name: str):
+    if name in ["info", "load", "save"]:
+        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 == "info":
+            return _info
+
+        if name == "load":
+            return _load
+
+        if name == "save":
+            return _save
+    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")
+
+
+def _load(
+    filepath: Union[str, Path],
+    out: Optional[Tensor] = None,
+    normalization: Union[bool, float, Callable] = True,
+    channels_first: bool = True,
+    num_frames: int = 0,
+    offset: int = 0,
+    filetype: Optional[str] = None,
+) -> Tuple[Tensor, int]:
+    raise RuntimeError("No audio I/O backend is available.")
+
+
+def _save(filepath: str, src: Tensor, sample_rate: int, precision: int = 16, channels_first: bool = True) -> None:
+    raise RuntimeError("No audio I/O backend is available.")
+
+
+def _info(filepath: str) -> torchaudio.AudioMetaData:
+    raise RuntimeError("No audio I/O backend is available.")

torchaudio/backend/soundfile_backend.py

@@ -0,0 +1,17 @@
+def __getattr__(name: str):
+    if name in ["info", "load", "save"]:
+        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)
+
+    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")

torchaudio/backend/_sox_io_backend.py → torchaudio/backend/sox_io_backend.py

@@ -6,8 +6,31 @@
 from torchaudio import AudioMetaData
 
 
+def __getattr__(name: str):
+    if name in ["info", "load", "save"]:
+        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 == "info":
+            return _info
+
+        if name == "load":
+            return _load
+
+        if name == "save":
+            return _save
+    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")
+
+
 @torchaudio._extension.fail_if_no_sox
-def info(
+def _info(
     filepath: str,
     format: Optional[str] = None,
 ) -> AudioMetaData:
@@ -34,7 +57,7 @@ def info(
 
 
 @torchaudio._extension.fail_if_no_sox
-def load(
+def _load(
     filepath: str,
     frame_offset: int = 0,
     num_frames: int = -1,
@@ -129,7 +152,7 @@ def load(
 
 
 @torchaudio._extension.fail_if_no_sox
-def save(
+def _save(
     filepath: str,
     src: torch.Tensor,
     sample_rate: int,