Add LocalGraphStore for distributed training

[ZhengHongming888]

This code belongs to the part of the whole distributed training for PyG.

This class extends from GraphStore and use the store dict to save the graph topology and also use edge_ids dict to save the global eids.

We also include the unit test under /test folder to show how the graph edge_index/ edge_ids will be saving/getting and verified.

Any comments please let us know. thanks

[codecov]

Codecov Report

Merging #7451 (7ccdd2d) into master (8770c93) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 7ccdd2d differs from pull request most recent head 640815d. Consider uploading reports for the commit 640815d to get more accurate results

@@            Coverage Diff             @@
##           master    #7451      +/-   ##
==========================================
+ Coverage   91.42%   91.43%   +0.01%     
==========================================
  Files         443      445       +2     
  Lines       24656    24689      +33     
==========================================
+ Hits        22542    22575      +33     
  Misses       2114     2114              

Impacted Files	Coverage Δ
torch_geometric/distributed/__init__.py	100.00% <100.00%> (ø)
torch_geometric/distributed/local_graph_store.py	100.00% <100.00%> (ø)

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[ZhengHongming888]

Thanks Matthias!