Annotate MelSpectrogram power as float only (pytorch#1572)

Mel scale frequency is in general defined/used on power spectrogram and as far as we know, it is not defined for raw (complex value) spectrogram. When `power=None` (when intermediate spectrogram is complex-valued), `MelSpectrogram` fails because of extra dimension (when using pseudo complex) or type mismatch between `torch.float` and torch.cfloat` (when using native complex).

This commit changes type annotation so that power is only float.

torchaudio/transforms.py

@@ -474,7 +474,7 @@ def __init__(self,
                  pad: int = 0,
                  n_mels: int = 128,
                  window_fn: Callable[..., Tensor] = torch.hann_window,
-                 power: Optional[float] = 2.,
+                 power: float = 2.,
                  normalized: bool = False,
                  wkwargs: Optional[dict] = None,
                  center: bool = True,