GPU loading + forward smoke test for HunyuanModel.
Loads the real HunyuanVideo checkpoint (~13B at bf16) via
HunyuanModel.__init__ and runs one transformer forward pass on
synthetic inputs. Hunyuan's transformer takes a slightly different
forward signature than Wan (no encoder_attention_mask, no
return_dict); this test mirrors the kwargs in
HunyuanModel._build_distill_input_kwargs.