Put structs common to backends in backend.common (#706)

It is strange that `sox_backend` depends on custom classes defined in `soudfile_backend`. This PR introduces `backend.common` module where these custom classes are defined and have both `sox_backend` and `soundfile_backend` depend on it.

torchaudio/backend/__init__.py

@@ -12,7 +12,7 @@ from .sox_backend import (
     get_sox_encoding_t,
     get_sox_bool,
 )
-from .soundfile_backend import (
+from .common import (
     SignalInfo,
     EncodingInfo,
 )

torchaudio/backend/common.py

+from typing import Any, Optional, Tuple
+
+
+class SignalInfo:
+    def __init__(self,
+                 channels: Optional[int] = None,
+                 rate: Optional[float] = None,
+                 precision: Optional[int] = None,
+                 length: Optional[int] = None) -> None:
+        self.channels = channels
+        self.rate = rate
+        self.precision = precision
+        self.length = length
+
+
+class EncodingInfo:
+    def __init__(self,
+                 encoding: Any = None,
+                 bits_per_sample: Optional[int] = None,
+                 compression: Optional[float] = None,
+                 reverse_bytes: Any = None,
+                 reverse_nibbles: Any = None,
+                 reverse_bits: Any = None,
+                 opposite_endian: Optional[bool] = None) -> None:
+        self.encoding = encoding
+        self.bits_per_sample = bits_per_sample
+        self.compression = compression
+        self.reverse_bytes = reverse_bytes
+        self.reverse_nibbles = reverse_nibbles
+        self.reverse_bits = reverse_bits
+        self.opposite_endian = opposite_endian

torchaudio/backend/soundfile_backend.py

 import os
-from typing import Any, Optional, Tuple
+from typing import Optional, Tuple
 
 import torch
 from torch import Tensor
@@ -8,6 +8,7 @@ from torchaudio._internal import (
     module_utils as _mod_utils,
     misc_ops as _misc_ops,
 )
+from .common import SignalInfo, EncodingInfo
 
 if _mod_utils.is_module_available('soundfile'):
     import soundfile
@@ -22,36 +23,6 @@ _subtype_to_precision = {
 }
 
 
-class SignalInfo:
-    def __init__(self,
-                 channels: Optional[int] = None,
-                 rate: Optional[float] = None,
-                 precision: Optional[int] = None,
-                 length: Optional[int] = None) -> None:
-        self.channels = channels
-        self.rate = rate
-        self.precision = precision
-        self.length = length
-
-
-class EncodingInfo:
-    def __init__(self,
-                 encoding: Any = None,
-                 bits_per_sample: Optional[int] = None,
-                 compression: Optional[float] = None,
-                 reverse_bytes: Any = None,
-                 reverse_nibbles: Any = None,
-                 reverse_bits: Any = None,
-                 opposite_endian: Optional[bool] = None) -> None:
-        self.encoding = encoding
-        self.bits_per_sample = bits_per_sample
-        self.compression = compression
-        self.reverse_bytes = reverse_bytes
-        self.reverse_nibbles = reverse_nibbles
-        self.reverse_bits = reverse_bits
-        self.opposite_endian = opposite_endian
-
-
 @_mod_utils.requires_module('soundfile')
 def load(filepath: str,
          out: Optional[Tensor] = None,

torchaudio/backend/sox_backend.py

@@ -8,7 +8,7 @@ from torchaudio._internal import (
     module_utils as _mod_utils,
     misc_ops as _misc_ops,
 )
-from .soundfile_backend import SignalInfo, EncodingInfo
+from .common import SignalInfo, EncodingInfo
 
 if _mod_utils.is_module_available('torchaudio._torchaudio'):
     from torchaudio import _torchaudio