Overload `Optim.optimize` for `::Node`

[MilesCranmer]

This depends on #27.

This implements a memory efficient Optim.optimize for ::Node set to the initial value. For example:

using Optim
using DynamicExpressions

operators = OperatorEnum(; binary_operators=[+, -, *, /], unary_operators=[cos, sin])

# Problem:
x1, x2, x3 = ntuple(i->Node(;feature=i), 3)
tree = cos(x1 * 3.2 - 5.8) * x2 - 0.8*x3*x3 - 0.2
truth(x) = cos(x[1] * 3.1 - 5.9) * x[2] - 2*x[3]^2 - 0.15

# Dataset:
X = randn(3, 32)
y = [truth(x) for x in eachcol(X)]

# Optimize!
optimize(tree -> sum(abs2, tree(X, operators) .- y), tree)

This gives us:

julia> tree
(((cos((x1 * 3.0999948526250973) - 5.900012476309413) * x2) - ((1.9999972274990503 * x3) * x3)) - 0.14998888911815988)

and is quite fast:

julia> @btime optimize(t -> sum(abs2, t(X) .- y), copy_node(tree))
  537.312 μs (4197 allocations: 357.98 KiB)
 * Status: success

 * Candidate solution
    Final objective value:     6.346103e-09

 * Found with
    Algorithm:     Nelder-Mead

 * Convergence measures
    √(Σ(yᵢ-ȳ)²)/n ≤ 1.0e-08

 * Work counters
    Seconds run:   0  (vs limit Inf)
    Iterations:    150
    f(x) calls:    263

(recall that this tree's structure is completely variable)

@AlCap23 @ChrisRackauckas maybe this is useful for your stuff too?

[github-actions[bot]]

Pull Request Test Coverage Report for Build 4915532924

• 100 of 140 (71.43%) changed or added relevant lines in 6 files are covered.
• 8 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-3.4%) to 85.372%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/base.jl	78	90	86.67%
src/ConstantOptimization.jl	0	28	0.0%
<!--	Total:	100	140	71.43%	-->

Files with Coverage Reduction	New Missed Lines	%
src/EvaluateEquation.jl	1	99.64%
src/Utils.jl	1	71.11%
src/InterfaceSymbolicUtils.jl	2	66.67%
src/precompile.jl	2	80.8%
src/SimplifyEquation.jl	2	86.42%
<!--	Total:	8		-->

Totals
Change from base Build 4824867612:	-3.4%
Covered Lines:	963
Relevant Lines:	1128

---

💛 - Coveralls

[github-actions[bot]]

Benchmark Results

	master	80c370fe8c11ba...	t[master]/t[80c370fe8c11ba...]
eval/ComplexF32/evaluation	7.27 ± 0.45 ms	7.22 ± 0.53 ms	1.01
eval/ComplexF64/evaluation	9.48 ± 0.7 ms	9.47 ± 0.69 ms	1
eval/Float32/derivative	10.6 ± 1.3 ms	10.5 ± 1.3 ms	1.01
eval/Float32/derivative_turbo	10.6 ± 1.2 ms	10.6 ± 1.3 ms	0.996
eval/Float32/evaluation	2.54 ± 0.23 ms	2.57 ± 0.23 ms	0.986
eval/Float32/evaluation_turbo	0.541 ± 0.022 ms	0.54 ± 0.022 ms	1
eval/Float64/derivative	13.3 ± 0.62 ms	13.3 ± 0.6 ms	0.994
eval/Float64/derivative_turbo	13.3 ± 0.6 ms	13.4 ± 0.54 ms	0.997
eval/Float64/evaluation	2.71 ± 0.25 ms	2.72 ± 0.23 ms	0.997
eval/Float64/evaluation_turbo	1.02 ± 0.059 ms	1.02 ± 0.059 ms	0.996
utils/combine_operators/break_sharing	0.0493 ± 0.0031 ms	0.0498 ± 0.0029 ms	0.991
utils/convert/break_sharing	27.8 ± 0.92 μs	27.6 ± 0.92 μs	1.01
utils/convert/preserve_sharing	0.126 ± 0.0024 ms	0.125 ± 0.0021 ms	1.01
utils/copy/break_sharing	28.6 ± 0.86 μs	28.1 ± 0.95 μs	1.02
utils/copy/preserve_sharing	0.126 ± 0.0023 ms	0.125 ± 0.0022 ms	1.01
utils/count_constants/break_sharing	10.3 ± 0.16 μs	10.5 ± 0.14 μs	0.985
utils/count_constants/preserve_sharing	0.111 ± 0.0021 ms	0.111 ± 0.0019 ms	0.998
utils/count_depth/break_sharing	17.2 ± 0.37 μs	12.7 ± 0.2 μs	1.35
utils/count_nodes/break_sharing	10.1 ± 0.15 μs	10.1 ± 0.15 μs	0.998
utils/count_nodes/preserve_sharing	0.113 ± 0.002 ms	0.114 ± 0.0019 ms	0.994
utils/get_set_constants!/break_sharing	0.0528 ± 0.00087 ms	0.053 ± 0.00081 ms	0.997
utils/get_set_constants!/preserve_sharing	0.316 ± 0.005 ms	0.316 ± 0.0048 ms	0.998
utils/has_constants/break_sharing	4.37 ± 0.23 μs	4.27 ± 0.22 μs	1.02
utils/has_operators/break_sharing	1.77 ± 0.02 μs	1.76 ± 0.02 μs	1.01
utils/hash/break_sharing	29.8 ± 0.45 μs	29.9 ± 0.44 μs	0.996
utils/hash/preserve_sharing	0.13 ± 0.0023 ms	0.131 ± 0.0042 ms	0.989
utils/index_constants/break_sharing	27.4 ± 0.68 μs	27.2 ± 0.58 μs	1.01
utils/index_constants/preserve_sharing	0.126 ± 0.0022 ms	0.125 ± 0.0022 ms	1.01
utils/is_constant/break_sharing	4.75 ± 0.22 μs	4.76 ± 0.22 μs	0.998
utils/simplify_tree/break_sharing	0.17 ± 0.015 ms	0.175 ± 0.015 ms	0.971
utils/simplify_tree/preserve_sharing	0.304 ± 0.018 ms	0.293 ± 0.017 ms	1.04
utils/string_tree/break_sharing	0.491 ± 0.0098 ms	0.491 ± 0.0091 ms	1
utils/string_tree/preserve_sharing	0.631 ± 0.011 ms	0.627 ± 0.012 ms	1.01
time_to_load	0.643 ± 0.0032 s	0.645 ± 0.0093 s	0.996

[ChrisRackauckas]

I don't think I've needed this but I can see how it could be useful.

[MilesCranmer]

The Enzyme interface would make this nicer: https://github.com/MilesCranmer/SymbolicRegression.jl/pull/254