add prod

[oxinabox]

I wanted to see if it was as simple as i thought it would be.
The answer is yes.

[codecov-io]

Codecov Report

Merging #385 (912663f) into master (76ef95c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #385   +/-   ##
=======================================
  Coverage   97.80%   97.80%           
=======================================
  Files          20       20           
  Lines        1547     1552    +5     
=======================================
+ Hits         1513     1518    +5     
  Misses         34       34           

Impacted Files	Coverage Δ
src/rulesets/Base/mapreduce.jl	100.00% <100.00%> (ø)

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[simeonschaub]

Needs complex tests as well

[willtebbutt]

Would you consider restricting the type of x to a subtype of AbstractArrays?

[oxinabox]

I just matched sum
I am happy to restrict to whatever

[willtebbutt]

Given that we've not tied this down, I propose StridedArray.

[sethaxen]

Can you comment on how this is relates to #335? Does this cover the same cases?

[oxinabox]

I was not aware of #335
This is definitely simpler that #335.
So I guess I am missing something

[oxinabox]

Yep the following fails:

julia>         test_rrule(prod, [2.0, 0.0, 1.0, 3.5])
test_rrule: prod at ([2.0, 0.0, 1.0, 3.5],): Test Failed at /Users/oxinabox/.julia/packages/ChainRulesTestUtils/AIKXr/src/check_result.jl:19
  Expression: isapprox(actual, expected; kwargs...)
   Evaluated: isapprox([0.0, NaN, 0.0, 0.0], [0.0, 0.8173013508577128, 0.0, 0.0]; rtol = 1.0e-9, atol = 1.0e-9)
Stacktrace:
 [1] check_equal(::Array{Float64,1}, ::Array{Float64,1}; kwargs::Base.Iterators.Pairs{Symbol,Float64,Tuple{Symbol,Symbol},NamedTuple{(:rtol, :atol),Tuple{Float64,Float64}}}) at /Users/oxinabox/.julia/packages/ChainRulesTestUtils/AIKXr/src/check_result.jl:19
 [2] macro expansion at /Users/oxinabox/.julia/packages/ChainRulesTestUtils/AIKXr/src/testers.jl:195 [inlined]
 [3] macro expansion at /usr/local/src/julia/julia-1.5/usr/share/julia/stdlib/v1.5/Test/src/Test.jl:1115 [inlined]
 [4] test_rrule(::typeof(prod), ::Array{Float64,1}; output_tangent::ChainRulesTestUtils.Auto, fdm::FiniteDifferences.AdaptedFiniteDifferenceMethod{5,1,FiniteDifferences.UnadaptedFiniteDifferenceMethod{7,5}}, check_inferred::Bool, fkwargs::NamedTuple{(),Tuple{}}, rtol::Float64, atol::Float64, kwargs::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}) at /Users/oxinabox/.julia/packages/ChainRulesTestUtils/AIKXr/src/testers.jl:160

Closing this infavor of #335