data.edge_subgraph and utility functions

[rusty1s]

• Adds data.edge_subgraph(subset)
• Adds mask_select(src, dim, mask) for 1-D masks
• Adds data.node_attrs() and data.edge_attrs()
• Adds data.generate_ids()

[codecov]

Codecov Report

Merging #6193 (55abde2) into master (865f7ce) will increase coverage by 0.03%.
The diff coverage is 98.30%.

@@            Coverage Diff             @@
##           master    #6193      +/-   ##
==========================================
+ Coverage   84.45%   84.48%   +0.03%     
==========================================
  Files         371      371              
  Lines       20677    20725      +48     
==========================================
+ Hits        17462    17509      +47     
- Misses       3215     3216       +1     

Impacted Files	Coverage Δ
torch_geometric/data/hetero_data.py	94.96% <90.90%> (-0.11%)	⬇️
torch_geometric/data/data.py	92.60% <100.00%> (+0.38%)	⬆️
torch_geometric/data/storage.py	79.63% <100.00%> (+0.25%)	⬆️
torch_geometric/utils/__init__.py	100.00% <100.00%> (ø)
torch_geometric/utils/mask.py	100.00% <100.00%> (ø)

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