Enable getting global indices from SamplerOutput

[zaristei]

The current implementation of SamplerOutput returns local indices of node ids in the node list for the row,col, batch, orig_row, and orig_col fields. This leads to some confusion for using the edges/seed nodes retrieved from the sampler. This PR adds some helper methods that will derive the global indices for these fields given the information already stored within the SamplerOutput.
Unittests have been also written for both homogeneous and heterogeneous outputs.

[Kh4L]

lgtm!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.85%. Comparing base (c211214) to head (37c97f5).
⚠️ Report is 137 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10200      +/-   ##
==========================================
- Coverage   86.11%   85.85%   -0.26%     
==========================================
  Files         496      501       +5     
  Lines       33655    34507     +852     
==========================================
+ Hits        28981    29625     +644     
- Misses       4674     4882     +208     

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[puririshi98]

LGTM but ill let @akihironitta or @rusty1s review and merge this one

[rusty1s]

What's the purpose?

[zaristei]

The purpose of these changes is to make it easier for users of SamplerOutput to access the global row/col ids, as well as global seed node ids from the sampler. Although the docstring indicates that the row and col fields are indices of nodes, it's not immediately obvious from that documentation that one can get the global indices by indexing into the nodes list, or that the same can be done for to get seed nodes from batch, for instance.

[puririshi98]

merging as per discussions with @rusty1s , these are heavily tested through NVIDIA already