fix: correct operator precedence in lvlb_weights calculation

[Mr-Neutr0n]

Bug

In the lvlb_weights calculation, the expression 1 - alphas_cumprod_prev / 1. - alphas_cumprod is parsed as 1 - (alphas_cumprod_prev / 1.) - alphas_cumprod due to Python operator precedence, instead of the intended (1 - alphas_cumprod_prev) / (1 - alphas_cumprod).

Fix

Added parentheses to ensure correct precedence matching the DDPM paper's posterior variance formula.

[Mr-Neutr0n]

This single-line fix corrects the operator precedence in the x0 parameterization branch of lvlb_weights. Without the parentheses around 1. - torch.Tensor(alphas_cumprod), the subtraction is evaluated after the multiplication, producing (2. * 1) - alphas_cumprod instead of the intended 2. * (1. - alphas_cumprod). The fix is clearly correct and has been open for about three months with no activity. Would a maintainer be able to take a look?

[Mr-Neutr0n]

Closing this one to tidy up my open pull requests.

It's been open around five months with no review activity, and I'd rather withdraw it than leave it sitting in your queue. Nothing needed from you, and no hard feelings at all.

If the fix is still wanted, this can be reopened, or I'm happy to redo it properly against current main. Apologies for the noise.