Enhancement on uniform random sampling of indices near zero.

[aschaffer]

This is a partial fix for #1979.

Specifically, given N = out-deg(v) and a random number r ∈ [0,1], one must obtain the equivalent discrete
index ∈ {0,1,...,N-1}. Previous implementation used an upper bound ubound = N-1 and a linear interpolation. As the issue above mentioned that approach creates problems near the (lower) boundary.

The fix uses a better bound, namely ubound = N and the discrete transformation: index = floor(r >= 1.0 ? N-1 : r*N).

Attached Mathematica plots show the graphs for, say, N = 13 and N=17.

This fix is not high priority for release 22-04, and can be included in the 22-06 release. Also, not all of the concerns formulated in the issue above are addressed by this PR. For example a uniform random generator callable from device is not yet available, but there are plans to perhaps expose something like that in raft.

[codecov-commenter]

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[ChuckHastings]

@gpucibot merge