GPU loading smoke test for WanCausalModel.
Verifies that WanCausalModel.__init__ resolves the
CausalWanTransformer3DModel class override and successfully loads
weights from the regular Wan2.1 1.3B checkpoint.
A real forward pass is intentionally omitted here: the causal
transformer requires per-frame timesteps, a block-causal attention
mask, and KV cache state that WanCausalModel.predict_noise_streaming
manages for production callers. PR 5 (per-method tests) exercises that
streaming forward path end-to-end.