Function std::mps_relu_backward_f32_chained_ab

int mps_relu_backward_f32_chained_ab(
    struct MpsBuf bufA,
    struct MpsBuf bufSelfGrad,
    &float out,
    unsigned long n,
)

Same as mps_relu_backward_f32_chained, but 'a' is ALSO a still-pending GPU buffer -- needed when relu's own input is itself an activation computed earlier in the SAME still-open batch (the fused forward+loss+backward training step). See gpu_mps.sc's comment.