If rng is set, then shuffle=true ought to be implicit

[ablaom]

In resampling strategies and in partition and perhaps elsewhere there is option to set shuffle=true, and also an option to set rng to integer or specific RNG object. If the latter is set, then shuffle=true ought to be forced. Currently shuffle has default false and if rng only is set then rng is ignored and no shuffling happens.

[ablaom]

Here specifically is the proposed logic for thepartition function:

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partition(rows::AbstractVector{Int}, fractions...; 
          shuffle=nothing, rng=Random.GLOBAL_RNG)

Splits the vector rows into a tuple of vectors whose lengths are
given by the corresponding fractions of length(rows). The last
fraction is not provided, as it is inferred from the preceding
ones. So, for example,

julia> partition(1:1000, 0.2, 0.7)
(1:200, 201:900, 901:1000)

Pre-shuffling of rows always occurs if rng is specified, unless
shuffle=false is also specified. Ifrng an integer, then
MersenneTwister(rng) is used as a random number generator; otherwise
some AbstractRNG object is expected.

To use the global random generator it suffices to specify
shuffle=true.

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Todo:

• partition (in MLJBase)
• CV resampling strategy
• Holdout resampling strategy
• StratifiedCV