`hann_window` does not obey COLA; therefore, it cannot be used with `istft`

🐛 Bug

In order to enable inversion of an STFT via the inverse STFT in istft, it is sufficient that the signal windowing obeys the constraint of “Constant OverLap Add” (COLA). This ensures that every point in the input data is equally weighted, thereby avoiding aliasing and allowing full reconstruction.
https://docs.scipy.org/doc/scipy/reference/generated/scipy.signal.check_COLA.html

Learn more: https://ccrma.stanford.edu/~jos/sasp/Mathematical_Definition_STFT.html#19930

To Reproduce

>>> from scipy import signal
>>> import torch
>>> signal.check_COLA(signal.hann(1024, sym=False), 1024, 1024 - 256)
True
>>> signal.check_COLA(torch.hann_window(1024).numpy(), 1024, 1024 - 256)
False
>>> window = torch.hann_window(1024).numpy()
>>> window[0] *= 0.5
>>> window[-1] *= 0.5
>>> signal.check_COLA(window, 1024, 1024 - 256)
False

Expected behavior

The expected behavior is that the hann window is implemented such that it is useable with istft.

Start at the torch.hann_window entry point and compare its output with scipy.signal.check_COLA using the parameters in the reproduction. Check how the window is used with istft; done when the Hann window satisfies the stated COLA case and the expected reconstruction behavior is covered by tests.