Flux 2: The shape of the latent argument is undocumented

The shape of the `latent` argument is not, as users might guess, `(1, 16, h//8, w//8)`, but `(1, 128, h//16, w//16)`.

This should be documented.

The issue names the Flux `latent` argument but no file or test. Start by locating the Flux API or documentation entry point that describes this argument, confirm the stated shape against the surrounding usage, and consider the work done when the `(1, 128, h//16, w//16)` shape is documented clearly.

Written by the indexing model from the issue text.