random_walk implementation

[rusty1s]

No description provided.

[codecov-commenter]

Codecov Report

Merging #21 (fa3a48c) into master (a7f20d4) will not change coverage.
The diff coverage is 100.00%.

❗ Current head fa3a48c differs from pull request most recent head 5f1cd19. Consider uploading reports for the commit 5f1cd19 to get more accurate results

@@            Coverage Diff            @@
##            master       #21   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         4    +3     
  Lines            5        32   +27     
=========================================
+ Hits             5        32   +27     

Impacted Files	Coverage Δ
pyg_lib/csrc/sampler/cpu/random_walk_kernel.cpp	100.00% <100.00%> (ø)
pyg_lib/csrc/sampler/cpu/utils.h	100.00% <100.00%> (ø)
pyg_lib/csrc/sampler/random_walk.cpp	100.00% <100.00%> (ø)

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[rusty1s]

@yaoyaowd This is now ready to review. GPU implementation will follow in a separate PR.

[rusty1s]

All review comments have been addressed. Thank you very much for these.