fix: NaN assertion uses .all() instead of .any()

[Mr-Neutr0n]

Bug

The NaN assertion assert not torch.isnan(x).all() only triggers when ALL values are NaN. The intended check is to catch ANY NaN values, which requires .any().

Fix

Changed .all() to .any() so the assertion catches any NaN values.

[Mr-Neutr0n]

Quick ping on the one-line fix in ddpm.py that corrects the NaN assertion from .all() to .any(). As written, np.all() only flags a NaN if every element is NaN, so a partially NaN tensor would pass through undetected. Would a maintainer be able to take a look?