Added a detailed warning if torch_cluster is not installed

[EdisonLeeeee]

Gives users a more detailed warning message if torch_cluster is not installed when using a related API, see also #7783

[rusty1s]

I like it, thanks :)

[codecov]

Codecov Report

Merging #7785 (2075124) into master (34890a5) will decrease coverage by 0.37%.
The diff coverage is 100.00%.

❗ Current head 2075124 differs from pull request most recent head c1e58a2. Consider uploading reports for the commit c1e58a2 to get more accurate results

@@            Coverage Diff             @@
##           master    #7785      +/-   ##
==========================================
- Coverage   91.95%   91.59%   -0.37%     
==========================================
  Files         453      453              
  Lines       25554    25547       -7     
==========================================
- Hits        23499    23399     -100     
- Misses       2055     2148      +93     

Impacted Files	Coverage Δ
torch_geometric/typing.py	89.85% <ø> (-10.15%)	⬇️
torch_geometric/nn/pool/__init__.py	72.72% <100.00%> (-1.19%)	⬇️
torch_geometric/utils/dropout.py	100.00% <100.00%> (ø)

... and 20 files with indirect coverage changes

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