This PR will fix #170 .
The previous gradient of a circuit is a vector of circuit parameter gradients. This causes the problem in #170 .
The gradient of a circuit now is a Yao Block instead of a vector type.

This solution is not yet perfect, the best return type should be Tagent{<:AbstractBlock}, because when you want to accumulate gradients, the block arithmetics does not do the right thing. As a result, you still need a patch for Zygote:

function Zygote.accum(a::AbstractBlock, b::AbstractBlock)
    dispatch(a, parameters(a) + parameters(b))
end