Update DistNeighborSampler for homo graphs [2/6]

[kgajdamo]

Changes made:

• added support for temporal sampling
• use torch.Tensors instead of numpy arrays
• move _sample_one_hop() from NeighborSampler to DistNeighborSampler
• do not go with disjoint flow in _sample() function - this is not needed because batch is calculated after
• added tests for node sampling and disjoint (works without DistNeighborLoader)
• added tests for node temporal sampling (works without DistNeighborLoader)
• some minor changes like changing variables names etc

This PR is based on the #8083, so both must be combined to pass the tests.

Other distributed PRs:
#8083
#8080
#8085

[codecov]

Codecov Report

Merging #8209 (765821c) into master (9ea2233) will increase coverage by 0.91%.
The diff coverage is 72.28%.

❗ Current head 765821c differs from pull request most recent head 6e1bf95. Consider uploading reports for the commit 6e1bf95 to get more accurate results

@@            Coverage Diff             @@
##           master    #8209      +/-   ##
==========================================
+ Coverage   87.17%   88.08%   +0.91%     
==========================================
  Files         473      473              
  Lines       28757    28774      +17     
==========================================
+ Hits        25068    25345     +277     
+ Misses       3689     3429     -260     

Files	Coverage Δ
torch_geometric/distributed/local_graph_store.py	96.93% <100.00%> (+1.06%)	⬆️
torch_geometric/sampler/neighbor_sampler.py	92.63% <87.50%> (+6.16%)	⬆️
...rch_geometric/distributed/dist_neighbor_sampler.py	58.00% <70.27%> (+58.00%)	⬆️

... and 6 files with indirect coverage changes

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[kgajdamo]

@rusty1s Thank you for the review :). One comment about these lines: they are common for both homo and hetero. In case of homo the attr.group_name will be None and therefore the condition isinstance(attr.group_name, NodeType) will never be met and node_attrs will be always empty. I will open a new PR with the previous version.