The istft is not entirely the inverse operation for stft when center=False.
hl = 512
n_fft = 1024

dummy_spec = torch.stft(torch.randn(1,1536), n_fft, hop_length=hl, center=False, onesided=True)
print(torch.istft(dummy_spec, n_fft, hop_length=hl, center=False, onesided=True).shape)

output >>> torch.Size([1, 1024]) 

Where the expected output shape should be (1, 1536).
In librosa, the same operation yield the correct result
dummy_lib_spec = librosa.stft(np.random.randn(1536), n_fft, hop_length=hl, center=False)
print(librosa.istft(dummy_lib_spec, hop_length=hl, center=False).shape)
output >>> (1536,)

I am using PyTorch 1.6.0