HeteroData: data.*_dict fix in case attribute does not exist

[rusty1s]

No description provided.

[codecov]

Codecov Report

Merging #3897 (2b09ff9) into master (3108d10) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3897      +/-   ##
==========================================
- Coverage   81.68%   81.66%   -0.02%     
==========================================
  Files         299      299              
  Lines       15208    15207       -1     
==========================================
- Hits        12422    12419       -3     
- Misses       2786     2788       +2     

Impacted Files	Coverage Δ
torch_geometric/data/hetero_data.py	92.71% <100.00%> (-0.03%)	⬇️
torch_geometric/transforms/gdc.py	77.94% <0.00%> (-1.03%)	⬇️

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[RexYing]

thx for helping to fix!