Support for multiple aggregations in SAGEConv

[lightaime]

This PR supports auto-infer the number of channels for multiple aggregations in SAGEConv.

Addresses item #5009 in #4712.

[codecov]

Codecov Report

Merging #5033 (d4ea1d0) into master (333d3d3) will decrease coverage by 1.87%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5033      +/-   ##
==========================================
- Coverage   84.85%   82.97%   -1.88%     
==========================================
  Files         332      332              
  Lines       18292    18300       +8     
==========================================
- Hits        15521    15184     -337     
- Misses       2771     3116     +345     

Impacted Files	Coverage Δ
torch_geometric/nn/aggr/multi.py	100.00% <100.00%> (+1.61%)	⬆️
torch_geometric/nn/conv/sage_conv.py	96.77% <100.00%> (+0.34%)	⬆️
torch_geometric/nn/models/dimenet_utils.py	0.00% <0.00%> (-75.52%)	⬇️
torch_geometric/nn/models/dimenet.py	14.51% <0.00%> (-53.00%)	⬇️
torch_geometric/nn/conv/utils/typing.py	81.25% <0.00%> (-17.50%)	⬇️
torch_geometric/profile/profile.py	32.94% <0.00%> (-15.30%)	⬇️
torch_geometric/nn/inits.py	67.85% <0.00%> (-7.15%)	⬇️
torch_geometric/nn/resolver.py	88.00% <0.00%> (-6.00%)	⬇️
torch_geometric/transforms/add_self_loops.py	94.44% <0.00%> (-5.56%)	⬇️
torch_geometric/io/tu.py	93.90% <0.00%> (-2.44%)	⬇️
... and 11 more

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[rusty1s]

I like it. Thank you!