Add padding capabilities to HeteroData.to_homogeneous()

[puririshi98]

useful for putting randomly generated FakeHeteroDataset data into a fused GNN like RGCNConv, otherwise the fakeheterodataset usually has x's w/ diff num of features and the resulting Data would not have any node features w/o this PR

[codecov]

Codecov Report

Merging #7374 (2964260) into master (2e64b0f) will decrease coverage by 0.01%.
The diff coverage is 82.35%.

❗ Current head 2964260 differs from pull request most recent head 979be58. Consider uploading reports for the commit 979be58 to get more accurate results

@@            Coverage Diff             @@
##           master    #7374      +/-   ##
==========================================
- Coverage   91.31%   91.30%   -0.01%     
==========================================
  Files         439      439              
  Lines       24494    24510      +16     
==========================================
+ Hits        22366    22379      +13     
- Misses       2128     2131       +3     

Impacted Files	Coverage Δ
torch_geometric/data/hetero_data.py	90.89% <82.35%> (-0.32%)	⬇️

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