Add missing doc-strings to nn.aggr

[rusty1s]

No description provided.

[codecov]

Codecov Report

Merging #5104 (de139fa) into master (4f392ac) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #5104   +/-   ##
=======================================
  Coverage   83.00%   83.00%           
=======================================
  Files         333      333           
  Lines       18365    18365           
=======================================
  Hits        15243    15243           
  Misses       3122     3122           

Impacted Files	Coverage Δ
torch_geometric/nn/aggr/__init__.py	100.00% <ø> (ø)
torch_geometric/nn/aggr/attention.py	100.00% <ø> (ø)
torch_geometric/nn/aggr/multi.py	100.00% <ø> (ø)
torch_geometric/nn/pool/__init__.py	89.65% <ø> (ø)
torch_geometric/nn/pool/glob.py	100.00% <ø> (ø)
torch_geometric/nn/aggr/basic.py	97.29% <100.00%> (ø)

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