fidelity gradient not working

[Roger-luo]

This works on 0.6 breaks on 0.7

julia> using Yao

julia> fidelity'(rand_state(10), rand_state(10)=>put(10, 1=>X))
(ArrayReg{1, ComplexF64, Array...}
    active qubits: 10/10, ArrayReg{1, ComplexF64, Array...}
    active qubits: 10/10 => Any[])