Add SignalInfo typedef, and extension module (#718)

This is a part of PRs to add new "sox_io" backend. https://github.com/pytorch/audio/pull/726

This PR adds `SignalInfo` structure, which is data exchange interface between Python and C++ in coming TorchScript-based sox IO backend.
For the case, where C++ extension is not available (i.e. Windows), this PR also adds dummy class and module that will be substituted.
This logic is implemented in `torchaudio.extension` moduel.

torchaudio/__init__.py

+from . import extension
 from torchaudio._internal import module_utils as _mod_utils
 from torchaudio import (
     compliance,

torchaudio/csrc/register.cpp

+#ifndef TORCHAUDIO_REGISTER_H
+#define TORCHAUDIO_REGISTER_H
+
+#include <torchaudio/csrc/typedefs.h>
+
+namespace torchaudio {
+namespace {
+
+static auto registerSignalInfo =
+    torch::class_<SignalInfo>("torchaudio", "SignalInfo")
+        .def(torch::init<int64_t, int64_t, int64_t>())
+        .def("get_sample_rate", &SignalInfo::getSampleRate)
+        .def("get_num_channels", &SignalInfo::getNumChannels)
+        .def("get_num_samples", &SignalInfo::getNumSamples);
+
+} // namespace
+} // namespace torchaudio
+#endif

torchaudio/csrc/typedefs.cpp

+#include <torchaudio/csrc/typedefs.h>
+
+namespace torchaudio {
+SignalInfo::SignalInfo(
+    const int64_t sample_rate_,
+    const int64_t num_channels_,
+    const int64_t num_samples_)
+    : sample_rate(sample_rate_),
+      num_channels(num_channels_),
+      num_samples(num_samples_){};
+
+int64_t SignalInfo::getSampleRate() const {
+  return sample_rate;
+}
+
+int64_t SignalInfo::getNumChannels() const {
+  return num_channels;
+}
+
+int64_t SignalInfo::getNumSamples() const {
+  return num_samples;
+}
+} // namespace torchaudio

torchaudio/csrc/typedefs.h

+#ifndef TORCHAUDIO_TYPDEFS_H
+#define TORCHAUDIO_TYPDEFS_H
+
+#include <torch/script.h>
+
+namespace torchaudio {
+struct SignalInfo : torch::CustomClassHolder {
+  int64_t sample_rate;
+  int64_t num_channels;
+  int64_t num_samples;
+
+  SignalInfo(
+      const int64_t sample_rate_,
+      const int64_t num_channels_,
+      const int64_t num_samples_);
+  int64_t getSampleRate() const;
+  int64_t getNumChannels() const;
+  int64_t getNumSamples() const;
+};
+
+} // namespace torchaudio
+
+#endif

torchaudio/extension/__init__.py

+from .extension import (
+    _init_extension,
+)
+
+_init_extension()
+
+del _init_extension

torchaudio/extension/extension.py

+import warnings
+import importlib
+from collections import namedtuple
+
+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)
+    else:
+        warnings.warn('torchaudio C++ extension is not available.')
+        _init_dummy_module()
+
+
+def _init_script_module(module):
+    path = importlib.util.find_spec(module).origin
+    torch.classes.load_library(path)
+    torch.ops.load_library(path)
+
+
+def _init_dummy_module():
+    class SignalInfo:
+        """Data class for audio format information
+
+        Used when torchaudio C++ extension is not available for annotating
+        sox_io backend functions so that torchaudio is still importable
+        without extension.
+        This class has to implement the same interface as C++ equivalent.
+        """
+        def __init__(self, sample_rate: int, num_channels: int, num_samples: int):
+            self.sample_rate = sample_rate
+            self.num_channels = num_channels
+            self.num_samples = num_samples
+
+        def get_sample_rate(self):
+            return self.sample_rate
+
+        def get_num_channels(self):
+            return self.num_channels
+
+        def get_num_samples(self):
+            return self.num_samples
+
+    DummyModule = namedtuple('torchaudio', ['SignalInfo'])
+    module = DummyModule(SignalInfo)
+    setattr(torch.classes, 'torchaudio', module)