Cube3D: use channels-first 1D latent (B,1,L) like Hunyuan3Dv2

Replaces the dummy trailing-dim latent with a channels-first 1D latent
(B, 1, num_tokens) and a dedicated latent_formats.Cube3D
(latent_channels=1, latent_dimensions=1). This mirrors the existing
native 3D model Hunyuan3Dv2's (B, C, L) convention and avoids
fix_empty_latent_channels truncating the token sequence (it narrows
dim=1 to latent_channels for empty latents). Requires no core sampler
changes: encode_model_conds sees a valid noise.shape[2].

- latent_formats.Cube3D added; wired into supported_models.Cube3D
- EmptyCubeLatent emits (B, 1, num_tokens)
- sample_cube takes T from x.shape[-1], returns (B, 1, T), and repeats
  conditioning to the latent batch size

Amp-Thread-ID: https://ampcode.com/threads/T-019ec361-addb-70d8-a74b-438ce8a1e096
Co-authored-by: Amp <amp@ampcode.com>

comfy/supported_models.py

@@ -1560,7 +1560,7 @@ class Cube3D(supported_models_base.BASE):
 
     sampling_settings = {}
 
-    latent_format = latent_formats.LatentFormat
+    latent_format = latent_formats.Cube3D
 
     memory_usage_factor = 1.0