Put libtorchaudio in lib directory (#1773)

Make the structure of library files somewhat similar to PyTorch core, which has the following pattern

```
torch/_C.so
torch/lib/libc10.so
torch/lib/libtorch.so
...
```

```
torchaudio/_torchaudio.so
torchaudio/lib/libtorchaudio.so
```

build_tools/setup_helpers/extension.py

@@ -44,7 +44,7 @@ _TORCH_CUDA_ARCH_LIST = os.environ.get('TORCH_CUDA_ARCH_LIST', None)
 
 def get_ext_modules():
     return [
-        Extension(name='torchaudio.libtorchaudio', sources=[]),
+        Extension(name='torchaudio.lib.libtorchaudio', sources=[]),
         Extension(name='torchaudio._torchaudio', sources=[]),
     ]
 

torchaudio/_extension.py

@@ -12,7 +12,7 @@ def _init_extension():
         return
 
     suffix = 'dll' if os.name == 'nt' else 'so'
-    path = Path(__file__).parent / f'libtorchaudio.{suffix}'
+    path = Path(__file__).parent / 'lib' / f'libtorchaudio.{suffix}'
     # In case `torchaudio` is deployed with `pex` format, this file does not exist.
     # In this case, we expect that `libtorchaudio` is available somewhere
     # in the search path of dynamic loading mechanism, and importing `_torchaudio`,

torchaudio/csrc/CMakeLists.txt

@@ -92,8 +92,8 @@ endif()
 
 install(
   TARGETS libtorchaudio
-  LIBRARY DESTINATION .
-  RUNTIME DESTINATION .
+  LIBRARY DESTINATION lib
+  RUNTIME DESTINATION lib  # For Windows
   )
 
 if (APPLE)

torchaudio/lib/.gitignore

+# For smooth build process, this `lib` directory has to exist.
+# git won't allow to have empty directory, so adding .gitignore
+# https://stackoverflow.com/a/932982
+*
+!.gitignore