Warn if the input dtype to TimeStretch is not complex

Author: mthrok

src/torchaudio/transforms/_transforms.py

@@ -1066,6 +1066,9 @@ def forward(self, complex_specgrams: Tensor, overriding_rate: Optional[float] =
                 Stretched spectrogram. The resulting tensor is of the corresponding complex dtype
                 as the input spectrogram, and the number of frames is changed to ``ceil(num_frame / rate)``.
         """
+        if not torch.is_complex(complex_specgrams):
+            warnings.warn("The input spectrogram must be complex type.", stacklevel=2)
+
         if overriding_rate is None:
             if self.fixed_rate is None:
                 raise ValueError("If no fixed_rate is specified, must pass a valid rate to the forward method.")