Add `class-resolver` for `Aggregation`

[cthoyt]

As a follow-up to #4687, this pull request adds a class-resolver for aggregations in torch_geometric.nn.aggr. It also adds a class docstring with an example on how to use it.

Note that class_resolver is a pure python package with no dependencies.

[codecov[bot]]

Codecov Report

Merging #4716 (1cb1f74) into master (cb92831) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4716   +/-   ##
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  Coverage   82.40%   82.40%           
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  Files         321      321           
  Lines       17207    17209    +2     
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+ Hits        14179    14181    +2     
  Misses       3028     3028           

Impacted Files	Coverage Δ
torch_geometric/nn/aggr/__init__.py	100.00% <100.00%> (ø)

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[cthoyt]

schönes Feierabend btw

[rusty1s]

@cthoyt We added basic support for aggregation_resolver in https://github.com/pyg-team/pytorch_geometric/pull/4749. Let me know once class-resolver is available on conda and I am happy to make the switch.