Fix: error on instance from batch_x missing from batch_y in nearest()

[Vuenc]

Fixed the error described in issue #163
I implemented the lenient approach from my reply there (error only on if some batch instance is present in batch_x, but missing from batch_y). I implemented this as-is for the CPU version, and an equivalent check on the ptr_x/ptr_y for the GPU version to catch the case where one of batch_x/batch_y is None.

I also checked if any of the other methods suffer from the same problem, but that's not the case. Only knn and radius take batch_x,batch_y arguments, and both seem to correctly handle empty instances (don't return any edge within an empty instance).

Additionally I implemented a check in nearest() if the batch_x, batch_y are sorted (as it's a general requirement of the method, and I also rely on it when calling torch.unique_consecutive()).

[codecov-commenter]

Codecov Report

Merging #168 (4c3ce21) into master (82e9df9) will increase coverage by 0.05%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master     #168      +/-   ##
==========================================
+ Coverage   98.83%   98.88%   +0.05%     
==========================================
  Files          10       10              
  Lines          86       90       +4     
==========================================
+ Hits           85       89       +4     
  Misses          1        1              

Impacted Files	Coverage Δ
torch_cluster/nearest.py	100.00% <100.00%> (ø)

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[rusty1s]

Thank you!