stft: Change require_complex warning to an error (#49022)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49022
**BC-breaking note**:
Previously torch.stft took an optional `return_complex` parameter that indicated whether the output would be a floating point tensor or a complex tensor. By default `return_complex` was False to be consistent with the previous behavior of torch.stft. This PR changes this behavior so `return_complex` is a required argument.
**PR Summary**:
* **#49022 stft: Change require_complex warning to an error**
Test Plan: Imported from OSS
Reviewed By: ngimel
Differential Revision: D25658906
Pulled By: mruberry
fbshipit-source-id: 11932d1102e93f8c7bd3d2d0b2a607fd5036ec5e
