Function gpu_batch_evolve 

pub fn gpu_batch_evolve(
    gpu: &GpuTrainingContext,
    unitaries_flat: &[Complex],
    rhos_flat: &[Complex],
    batch_count: usize,
) -> Vec<Complex>

GPU-accelerated batched evolve for positions where the thermodynamic gate passes.

Takes a batch of (unitary, density_matrix) pairs and computes ρ’ = U·ρ·U† on GPU. Returns evolved density matrices.