[Cherry-picked 0.9] Ignore return_complex when returning real-valued tensor in spectrogram.

[mthrok]

In 0.9.0, we are adding return_complex=False to spectrogram operation, so that users can choose whether they receive the resulting complex-valued tensor in pseudo complex tensor (real dtype with an extra dimension for real/imaginary parts) or native complex tensor (torch.cfloat / torch.cdouble).

Spectrogram operation returns raw, complex-valued tensor only when power=None. If power has some number, it returns the corresponding norm of the raw complex element. (such as power spectrogram) In this case, the value is always represented with real type, and there is no need to represent them in complex number.

The current version (0.9-RC1) checks if return_complex=True when power is not non. However, this does not align well with the use case. Users expecting power-spectrogram should not worry about this return_complex argument, which has its purpose in a different operation mode of spectrogram op. Also in #1549, I am switching the default value of return_complex to True and this means that by default, users who want power spectrogram has to provide return_complex=False additionally. This is totally unnecessary and avoidable. Therefore we should only care about the value of return_complex, when power=None in spectrogram op.

[mthrok]

@carolineechen Can you review this? The return_complex argument here is very similar to beta parameter of F.resample, which is only effective when dealing with Kaiser window.

[carolineechen]

looks good