GlobalPooling and graph-level to_hetero support

[rusty1s]

Fixes #4565

[codecov]

Codecov Report

Merging #4582 (cd74536) into master (d268e86) will increase coverage by 0.02%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master    #4582      +/-   ##
==========================================
+ Coverage   82.77%   82.79%   +0.02%     
==========================================
  Files         315      315              
  Lines       16512    16590      +78     
==========================================
+ Hits        13667    13735      +68     
- Misses       2845     2855      +10     

Impacted Files	Coverage Δ
torch_geometric/nn/glob/glob.py	86.84% <76.19%> (-13.16%)	⬇️
torch_geometric/nn/to_hetero_transformer.py	95.20% <91.89%> (-1.06%)	⬇️
torch_geometric/nn/fx.py	89.04% <93.54%> (+0.51%)	⬆️
torch_geometric/nn/glob/__init__.py	100.00% <100.00%> (ø)

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