Function std::mps_matmul_atb_f32_chained_ab

int mps_matmul_atb_f32_chained_ab(
    struct MpsBuf bufA,
    struct MpsBuf bufDC,
    &float out,
    unsigned long M,
    unsigned long K,
    unsigned long N,
)

Same as mps_matmul_atb_f32_chained, but 'a' is ALSO a still-pending GPU buffer (not a plain CPU array) -- needed when the matmul's first operand is itself an activation computed earlier in the SAME still-open batch (e.g. dW2 = H^T . dY in the fused forward+loss+backward training step, where H hasn't been read back to CPU yet either). See gpu_mps.sc's comment on the implementation.