1775: Fix a doctest failure r=mcabbott a=mcognetta

This fixes a doctest failure that seems to be causing builds to fail. The issue seems to be that gradient now returns floats, even if the inputs are integers.

See the following stack trace:

```
┌ Error: doctest failure in ~/work/Flux.jl/Flux.jl/docs/src/models/basics.md:25-30
│ 
│ ```jldoctest basics
│ julia> f(x, y) = sum((x .- y).^2);
│ 
│ julia> gradient(f, [2, 1], [2, 0])
│ ([0, 2], [0, -2])
│ ```
│ 
│ Subexpression:
│ 
│ gradient(f, [2, 1], [2, 0])
│ 
│ Evaluated output:
│ 
│ ([0.0, 2.0], [-0.0, -2.0])
│ 
│ Expected output:
│ 
│ ([0, 2], [0, -2])
│ 
│   diff =
│    Warning: Diff output requires color.
│    ([0, 2], [0, -2])([0.0, 2.0], [-0.0, -2.0])
└ @ Documenter.DocTests ~/.julia/packages/Documenter/bFHi4/src/DocTests.jl:373
┌ Error: doctest failure in ~/work/Flux.jl/Flux.jl/docs/src/models/basics.md:37-56
│ 
│ ```jldoctest basics
│ julia> x = [2, 1];
│ 
│ julia> y = [2, 0];
│ 
│ julia> gs = gradient(params(x, y)) do
│          f(x, y)
│        end
│ Grads(...)
│ 
│ julia> gs[x]
│ 2-element Vector{Int64}:
│  0
│  2
│ 
│ julia> gs[y]
│ 2-element Vector{Int64}:
│   0
│  -2
│ ```
│ 
│ Subexpression:
│ 
│ gs[x]
│ 
│ Evaluated output:
│ 
│ 2-element Vector{Float64}:
│  0.0
│  2.0
│ 
│ Expected output:
│ 
│ 2-element Vector{Int64}:
│  0
│  2
│ 
│   diff =
│    Warning: Diff output requires color.
│    2-element Vector{Int64}:
│     0
│     2Vector{Float64}:
│     0.0
│     2.0
└ @ Documenter.DocTests ~/.julia/packages/Documenter/bFHi4/src/DocTests.jl:373
┌ Error: doctest failure in ~/work/Flux.jl/Flux.jl/docs/src/models/basics.md:37-56
│ 
│ ```jldoctest basics
│ julia> x = [2, 1];
│ 
│ julia> y = [2, 0];
│ 
│ julia> gs = gradient(params(x, y)) do
│          f(x, y)
│        end
│ Grads(...)
│ 
│ julia> gs[x]
│ 2-element Vector{Int64}:
│  0
│  2
│ 
│ julia> gs[y]
│ 2-element Vector{Int64}:
│   0
│  -2
│ ```
│ 
│ Subexpression:
│ 
│ gs[y]
│ 
│ Evaluated output:
│ 
│ 2-element Vector{Float64}:
│  -0.0
│  -2.0
│ 
│ Expected output:
│ 
│ 2-element Vector{Int64}:
│   0
│  -2
│ 
│   diff =
│    Warning: Diff output requires color.
│    2-element Vector{Int64}:
│      0
│     -2Vector{Float64}:
│     -0.0
│     -2.0
└ @ Documenter.DocTests ~/.julia/packages/Documenter/bFHi4/src/DocTests.jl:373
┌ Error: Doctesting failed
│   exception =
│    `makedocs` encountered a doctest error. Terminating build
│    Stacktrace:
│      [1] error(s::String)
│        @ Base ./error.jl:33
│      [2] runner(#unused#::Type{Documenter.Builder.Doctest}, doc::Documenter.Documents.Document)
│        @ Documenter.Builder ~/.julia/packages/Documenter/bFHi4/src/Builder.jl:217
│      [3] dispatch(#unused#::Type{Documenter.Builder.DocumentPipeline}, x::Documenter.Documents.Document)
│        @ Documenter.Utilities.Selectors ~/.julia/packages/Documenter/bFHi4/src/Utilities/Selectors.jl:170
│      [4] #2
│        @ ~/.julia/packages/Documenter/bFHi4/src/Documenter.jl:249 [inlined]
│      [5] cd(f::Documenter.var"#2#3"{Documenter.Documents.Document}, dir::String)
│        @ Base.Filesystem ./file.jl:106
│      [6] makedocs(; debug::Bool, format::Documenter.Writers.HTMLWriter.HTML, kwargs::Base.Iterators.Pairs{Symbol, Any, NTuple{6, Symbol}, NamedTuple{(:root, :source, :sitename, :doctest, :modules, :doctestfilters), Tuple{String, String, String, Symbol, Vector{Module}, Vector{Regex}}}})
│        @ Documenter ~/.julia/packages/Documenter/bFHi4/src/Documenter.jl:248
│      [7] (::Documenter.var"#all_doctests#35"{Bool, Vector{Regex}, Vector{Module}})()
│        @ Documenter ~/.julia/packages/Documenter/bFHi4/src/Documenter.jl:815
│      [8] macro expansion
│        @ ~/.julia/packages/Documenter/bFHi4/src/Documenter.jl:836 [inlined]
│      [9] macro expansion
│        @ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Test/src/Test.jl:1151 [inlined]
│     [10] doctest(source::String, modules::Vector{Module}; fix::Bool, testset::String, doctestfilters::Vector{Regex})
│        @ Documenter ~/.julia/packages/Documenter/bFHi4/src/Documenter.jl:836
│     [11] doctest(package::Module; manual::Bool, testset::Nothing, kwargs::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
│        @ Documenter ~/.julia/packages/Documenter/bFHi4/src/Documenter.jl:771
│     [12] doctest(package::Module)
│        @ Documenter ~/.julia/packages/Documenter/bFHi4/src/Documenter.jl:758
│     [13] top-level scope
│        @ none:7
│     [14] eval
│        @ ./boot.jl:360 [inlined]
│     [15] exec_options(opts::Base.JLOptions)
│        @ Base ./client.jl:261
│     [16] _start()
│        @ Base ./client.jl:485
└ @ Documenter ~/.julia/packages/Documenter/bFHi4/src/Documenter.jl:825
Doctests: Flux: Test Failed at /home/runner/.julia/packages/Documenter/bFHi4/src/Documenter.jl:836
  Expression: all_doctests()
Stacktrace:
 [1] macro expansion
   @ ~/.julia/packages/Documenter/bFHi4/src/Documenter.jl:836 [inlined]
 [2] macro expansion
   @ /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Test/src/Test.jl:1151 [inlined]
 [3] doctest(source::String, modules::Vector{Module}; fix::Bool, testset::String, doctestfilters::Vector{Regex})
   @ Documenter ~/.julia/packages/Documenter/bFHi4/src/Documenter.jl:836
Test Summary:  | Fail  Total
Doctests: Flux |    1      1
ERROR: Some tests did not pass: 0 passed, 1 failed, 0 errored, 0 broken.
Error: Process completed with exit code 1.
```

Co-authored-by: Marco Cognetta <cognetta.marco@gmail.com>
Co-authored-by: Marco <cognetta.marco@gmail.com>