Dicts in struct fields don't propagate gradients

[SBuercklin]

If we insert a Dict into a struct field and do some calculations with that Dict, we get a nothing derivative with respect to that field.

Example is on 0.6.29. I included a second field to show that gradients still accumulate for the other fields.

struct Container
    x
    y
end
f_dict(d,y) = d[:a]^2 + y^3
f_container(c) = c.x[:a]^2 + c.y^3

d = Dict(:a => 3)
c = Container(d, 4.0)

Zygote.gradient(f_dict, d, 4.0)
# (Dict{Any, Any}(:a => 6.0), 48.0)

Zygote.gradient(f_container, c)
# ((x = nothing, y = 48.0),)

[SBuercklin]

Duplicate of #717

[CarloLucibello]

@SBuercklin we have many Dict-related issues
dictionary
I suggest to use NamedTuple instead of dictionary whenever possible