Add random_walk_pe

[aurorarossi]

This PR add the RandomWalkPE of issue #190

[CarloLucibello]

Let's define the more basic function

random_walk_pe(g::GNNGraph, walk_length::Int)

that returns the positional encoding. Then the user can add it to the graph or do anything with it.

[codecov]

Codecov Report

Merging #273 (19a893b) into master (5b23e9c) will increase coverage by 0.04%.
The diff coverage is 93.75%.

❗ Current head 19a893b differs from pull request most recent head 8c14883. Consider uploading reports for the commit 8c14883 to get more accurate results

@@            Coverage Diff             @@
##           master     #273      +/-   ##
==========================================
+ Coverage   82.11%   82.15%   +0.04%     
==========================================
  Files          17       17              
  Lines        1923     1939      +16     
==========================================
+ Hits         1579     1593      +14     
- Misses        344      346       +2     

Impacted Files	Coverage Δ
src/GNNGraphs/transform.jl	96.93% <93.75%> (-0.55%)	⬇️

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[CarloLucibello]

nice, thanks