Support for multiple aggregations within MessagePassing

[rusty1s]

class MyConv(MessagePassing):
    def __init__(self, aggr=['add', 'mean', 'max']):
        super.__init__(aggr=aggr)

[codecov]

Codecov Report

Merging #4350 (548be72) into master (222b42f) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4350      +/-   ##
==========================================
+ Coverage   82.29%   82.31%   +0.02%     
==========================================
  Files         310      310              
  Lines       15913    15932      +19     
==========================================
+ Hits        13096    13115      +19     
  Misses       2817     2817              

Impacted Files	Coverage Δ
torch_geometric/nn/conv/message_passing.py	99.40% <100.00%> (+0.03%)	⬆️

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