Re-factor ClusterLoader + Integrate pyg-lib metis computation

[rusty1s]

No description provided.

[codecov]

Codecov Report

Merging #7416 (81507de) into master (627d310) will decrease coverage by 0.38%.
The diff coverage is 96.26%.

❗ Current head 81507de differs from pull request most recent head 9ff4d6a. Consider uploading reports for the commit 9ff4d6a to get more accurate results

@@            Coverage Diff             @@
##           master    #7416      +/-   ##
==========================================
- Coverage   91.71%   91.33%   -0.38%     
==========================================
  Files         440      441       +1     
  Lines       24580    24649      +69     
==========================================
- Hits        22543    22514      -29     
- Misses       2037     2135      +98     

Impacted Files	Coverage Δ
torch_geometric/loader/cluster.py	96.47% <95.32%> (-0.04%)	⬇️
torch_geometric/data/data.py	89.80% <100.00%> (-0.05%)	⬇️
torch_geometric/data/hetero_data.py	90.83% <100.00%> (-0.06%)	⬇️
torch_geometric/typing.py	100.00% <100.00%> (ø)
torch_geometric/utils/map.py	100.00% <100.00%> (ø)

... and 20 files with indirect coverage changes

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