Behavior on `vcat` of scalars

Here's an inconsistency I discovered based on two ways of constructing vectors from floats:
```julia
julia> using Tracker

julia> f1(x) = [x, x];

julia> f2(x) = vcat(x, x);

julia> x = 1.0;

julia> dy = ones(2);

julia> y1, pb1 = Tracker.forward(f1, x);

julia> y2, pb2 = Tracker.forward(f2, x);

julia> y1 == y2
true

julia> pb1(dy) == pb2(dy)
false

julia> pb1(dy) # returns a scalar
(2.0,)

julia> pb2(dy) # returns an array
([2.0],)
```

Reproduce the examples using Tracker.forward with f1(x) = [x, x] and f2(x) = vcat(x, x), then trace how each expression constructs its pullback. Compare the returned scalar and array gradients and determine the intended consistent behavior; the issue does not name source files or tests.

Written by the indexing model from the issue text.