Function k4_ssd_state_passing 

pub fn k4_ssd_state_passing(
    intra_chunk_state_bnhpr: Tensor<5>,
    da_chunk_end_bhn: Tensor<3>,
    initial_state_bhpr: Tensor<4>,
) -> (Tensor<5>, Tensor<4>)

Based on the Kernel 4 Triton reference _state_passing_fwd_kernel (ssd_state_passing.py).

Returns:

• chunk_input_state_bnhpr [used in K5][!].
• final_state_bhpr [final output].