fix sqrt_1m_alpha_bar

Author: vpj

labml_nn/diffusion/stable_diffusion/sampler/ddpm.py

@@ -68,7 +68,7 @@ def __init__(self, model: LatentDiffusion):
             # $\sqrt{\bar\alpha}$
             self.sqrt_alpha_bar = alpha_bar ** .5
             # $\sqrt{1 - \bar\alpha}$
-            self.sqrt_1m_alpha_bar = alpha_bar ** .5
+            self.sqrt_1m_alpha_bar = (1. - alpha_bar) ** .5
             # $\frac{1}{\sqrt{\bar\alpha_t}}$
             self.sqrt_recip_alpha_bar = alpha_bar ** -.5
             # $\sqrt{\frac{1}{\bar\alpha_t} - 1}$