[BC-Breaking] Default to native complex type when returning raw spectrogram

Part of #1337 .

- This code changes the return type of spectrogram to be native complex dtype,
when (and only when) returning raw (complex-valued) spectrogram.
- Change `return_complex=False` to `return_complex=True` in spectrogram ops.
- `return_complex` is only effective when `power` is `None`. It is ignored for
cases where `power` is not `None`. Because the returned Tensor is power spectrogram,
which is real-valued Tensors.

Author: mthrok

test/torchaudio_unittest/functional/torchscript_consistency_impl.py

@@ -51,20 +51,34 @@ def _assert_consistency_complex(self, func, tensor, test_pseudo_complex=False):
 
         self.assertEqual(ts_output, output)
 
-    def test_spectrogram(self):
+    def test_spectrogram_complex(self):
         def func(tensor):
             n_fft = 400
             ws = 400
             hop = 200
             pad = 0
             window = torch.hann_window(ws, device=tensor.device, dtype=tensor.dtype)
-            power = 2.
+            power = None
             normalize = False
             return F.spectrogram(tensor, pad, window, n_fft, hop, ws, power, normalize)
 
         tensor = common_utils.get_whitenoise()
         self._assert_consistency(func, tensor)
 
+    def test_spectrogram_real(self):
+        def func(tensor):
+            n_fft = 400
+            ws = 400
+            hop = 200
+            pad = 0
+            window = torch.hann_window(ws, device=tensor.device, dtype=tensor.dtype)
+            power = 2.
+            normalize = False
+            return F.spectrogram(tensor, pad, window, n_fft, hop, ws, power, normalize, return_complex=False)
+
+        tensor = common_utils.get_whitenoise()
+        self._assert_consistency(func, tensor)
+
     @skipIfRocm
     def test_griffinlim(self):
         def func(tensor):

torchaudio/functional/functional.py

@@ -49,7 +49,7 @@ def spectrogram(
         center: bool = True,
         pad_mode: str = "reflect",
         onesided: bool = True,
-        return_complex: bool = False,
+        return_complex: bool = True,
 ) -> Tensor:
     r"""Create a spectrogram or a batch of spectrograms from a raw audio signal.
     The spectrogram can be either magnitude-only or complex.

torchaudio/transforms.py

@@ -80,7 +80,7 @@ def __init__(self,
                  center: bool = True,
                  pad_mode: str = "reflect",
                  onesided: bool = True,
-                 return_complex: bool = False) -> None:
+                 return_complex: bool = True) -> None:
         super(Spectrogram, self).__init__()
         self.n_fft = n_fft
         # number of FFT bins. the returned STFT result will have n_fft // 2 + 1