Add IndexToMask and MaskToIndex transforms

[hatemhelal]

Both of these transforms will try to infer the right attributes to convert.

• MaskToIndex matches any attributes that end with _mask
• IndexToMask matches any attributes that end with _indices.

For the latter case, it would be a problem to automatically convert edge_index so I went with this suffix.

[codecov]

Codecov Report

Merging #5375 (faf5e97) into master (36244de) will increase coverage by 0.05%.
The diff coverage is 96.55%.

@@            Coverage Diff             @@
##           master    #5375      +/-   ##
==========================================
+ Coverage   83.39%   83.45%   +0.05%     
==========================================
  Files         347      348       +1     
  Lines       18849    18907      +58     
==========================================
+ Hits        15720    15778      +58     
  Misses       3129     3129              

Impacted Files	Coverage Δ
torch_geometric/transforms/mask.py	96.49% <96.49%> (ø)
torch_geometric/transforms/__init__.py	100.00% <100.00%> (ø)
torch_geometric/data/storage.py	82.07% <0.00%> (+0.31%)	⬆️
torch_geometric/utils/mask.py	100.00% <0.00%> (+9.09%)	⬆️

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[rusty1s]

This mostly looks good, thank you! Left some nitpicky comments. One thing I would like to add is HeteroData support. If this is too much to handle, we can add it in a follow-up PR.

[rusty1s]

Hey @hatemhelal. Can you give me write access to your fork/your PR?

[rusty1s]

Thank you!