Edit ADTypes docs

[JeffFessler]

This line of the docs claims that one is required to load ForwardDiff (e.g.) "beforehand"
but the autodiff=... option works even if the called never loads ForwardDiff,
presumably because ForwardDiff is already in the Project.toml file for Optim.jl.

Indeed, the example that is at the end of this docs page
https://julianlsolvers.github.io/Optim.jl/dev/user/gradientsandhessians/
does not load ForwardDiff, reinforcing that it is not "required."

So I propose to remove the claim that it is "required."
Perhaps there is some nuance here that I am missing, in which case the line should be edited to convey that nuance.

[github-actions]

Benchmark Results (Julia vlts)

Time benchmarks

	master	9643511...	master / 9643511...
multivariate/solvers/first_order/AdaMax	0.545 ± 0.01 ms	0.545 ± 0.01 ms	1 ± 0.026
multivariate/solvers/first_order/Adam	0.544 ± 0.01 ms	0.544 ± 0.0095 ms	1 ± 0.025
multivariate/solvers/first_order/BFGS	0.266 ± 0.009 ms	0.266 ± 0.0086 ms	0.997 ± 0.047
multivariate/solvers/first_order/ConjugateGradient	0.177 ± 0.0029 ms	0.178 ± 0.0028 ms	0.994 ± 0.023
multivariate/solvers/first_order/GradientDescent	1.57 ± 0.013 ms	1.58 ± 0.011 ms	0.994 ± 0.011
multivariate/solvers/first_order/LBFGS	0.236 ± 0.0084 ms	0.239 ± 0.0085 ms	0.991 ± 0.05
multivariate/solvers/first_order/MomentumGradientDescent	2.19 ± 0.015 ms	2.21 ± 0.016 ms	0.99 ± 0.0099
multivariate/solvers/first_order/NGMRES	0.436 ± 0.012 ms	0.441 ± 0.012 ms	0.989 ± 0.038
time_to_load	0.282 ± 0.0015 s	0.279 ± 0.0021 s	1.01 ± 0.0093

Memory benchmarks

	master	9643511...	master / 9643511...
multivariate/solvers/first_order/AdaMax	0.34 k allocs: 7.28 kB	0.34 k allocs: 7.28 kB	1
multivariate/solvers/first_order/Adam	0.34 k allocs: 7.28 kB	0.34 k allocs: 7.28 kB	1
multivariate/solvers/first_order/BFGS	0.281 k allocs: 13.3 kB	0.281 k allocs: 13.3 kB	1
multivariate/solvers/first_order/ConjugateGradient	0.294 k allocs: 14.7 kB	0.294 k allocs: 14.7 kB	1
multivariate/solvers/first_order/GradientDescent	1.73 k allocs: 0.0757 MB	1.73 k allocs: 0.0757 MB	1
multivariate/solvers/first_order/LBFGS	0.298 k allocs: 15 kB	0.298 k allocs: 15 kB	1
multivariate/solvers/first_order/MomentumGradientDescent	2.07 k allocs: 0.081 MB	2.07 k allocs: 0.081 MB	1
multivariate/solvers/first_order/NGMRES	1.39 k allocs: 0.114 MB	1.39 k allocs: 0.114 MB	1
time_to_load	0.153 k allocs: 14.5 kB	0.153 k allocs: 14.5 kB	1

A plot of the benchmark results has been uploaded as an artifact at .

[github-actions]

Benchmark Results (Julia v1)

Time benchmarks

	master	9643511...	master / 9643511...
multivariate/solvers/first_order/AdaMax	0.586 ± 0.0099 ms	0.586 ± 0.01 ms	1 ± 0.024
multivariate/solvers/first_order/Adam	0.586 ± 0.01 ms	0.586 ± 0.01 ms	1 ± 0.024
multivariate/solvers/first_order/BFGS	0.204 ± 0.0044 ms	0.204 ± 0.0041 ms	1 ± 0.03
multivariate/solvers/first_order/ConjugateGradient	0.048 ± 0.00073 ms	0.0479 ± 0.00065 ms	1 ± 0.02
multivariate/solvers/first_order/GradientDescent	1.55 ± 0.02 ms	1.55 ± 0.019 ms	1 ± 0.018
multivariate/solvers/first_order/LBFGS	0.206 ± 0.0046 ms	0.205 ± 0.004 ms	1 ± 0.03
multivariate/solvers/first_order/MomentumGradientDescent	2.24 ± 0.028 ms	2.24 ± 0.029 ms	0.999 ± 0.018
multivariate/solvers/first_order/NGMRES	0.52 ± 0.011 ms	0.52 ± 0.012 ms	1 ± 0.031
time_to_load	0.304 ± 0.0025 s	0.305 ± 0.0029 s	0.997 ± 0.012

Memory benchmarks

	master	9643511...	master / 9643511...
multivariate/solvers/first_order/AdaMax	0.354 k allocs: 7.19 kB	0.354 k allocs: 7.19 kB	1
multivariate/solvers/first_order/Adam	0.354 k allocs: 7.19 kB	0.354 k allocs: 7.19 kB	1
multivariate/solvers/first_order/BFGS	0.266 k allocs: 9.7 kB	0.266 k allocs: 9.7 kB	1
multivariate/solvers/first_order/ConjugateGradient	0.108 k allocs: 4.89 kB	0.108 k allocs: 4.89 kB	1
multivariate/solvers/first_order/GradientDescent	2 k allocs: 0.0708 MB	2 k allocs: 0.0708 MB	1
multivariate/solvers/first_order/LBFGS	0.336 k allocs: 13.7 kB	0.336 k allocs: 13.7 kB	1
multivariate/solvers/first_order/MomentumGradientDescent	2.39 k allocs: 0.0793 MB	2.39 k allocs: 0.0793 MB	1
multivariate/solvers/first_order/NGMRES	2.62 k allocs: 0.129 MB	2.62 k allocs: 0.129 MB	1
time_to_load	0.145 k allocs: 11 kB	0.145 k allocs: 11 kB	1

A plot of the benchmark results has been uploaded as an artifact at .