Require sorted neighborhoods according to time in temporal sampling

[ZenoTan]

As discussed offline, we need to unify the implementation for temporal and non-temporal sampling.
This PR updates the neighbor sampling kernel by deciding the range of sampling based on seed node time. We assume we have sorted each neighborhood by time. This assumption will be guaranteed on pyg side in the follow-up PRs.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 85.71429% with 4 lines in your changes missing coverage. Please review.

Project coverage is 84.70%. Comparing base (34c08c9) to head (79c7aae).
Report is 180 commits behind head on master.

Files	Patch %	Lines
pyg_lib/csrc/sampler/cpu/neighbor_kernel.cpp	85.71%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #108      +/-   ##
==========================================
+ Coverage   78.50%   84.70%   +6.20%     
==========================================
  Files          16       16              
  Lines         428      412      -16     
==========================================
+ Hits          336      349      +13     
+ Misses         92       63      -29     

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[rusty1s]

This is amazing, thank you! Also added a note to the documentation.