Add new cat option to the aggr argument for HeteroConv

[DylanSand]

This new aggregation type will allow users to have the option of having the features from different edge types be concatenated together during the grouping stage of HeteroConv's forward function.

[codecov]

Codecov Report

Merging #6634 (0726163) into master (a730437) will increase coverage by 0.00%.
The diff coverage is 91.66%.

❗ Current head 0726163 differs from pull request most recent head 1d70f56. Consider uploading reports for the commit 1d70f56 to get more accurate results

@@           Coverage Diff           @@
##           master    #6634   +/-   ##
=======================================
  Coverage   87.55%   87.56%           
=======================================
  Files         422      422           
  Lines       22879    22886    +7     
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+ Hits        20032    20039    +7     
  Misses       2847     2847           

Impacted Files	Coverage Δ
torch_geometric/utils/scatter.py	72.72% <ø> (ø)
torch_geometric/nn/conv/hgt_conv.py	96.07% <50.00%> (-0.93%)	⬇️
torch_geometric/data/data.py	88.86% <100.00%> (+0.02%)	⬆️
torch_geometric/data/hetero_data.py	90.63% <100.00%> (+0.29%)	⬆️

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