Add SGFormer model and example

[xnuohz]

Issue

• From Graph Transformer Enhancement #9751
• Adds an example for training SGFormer model on ogbn-arxiv dataset.

Usage

python examples/sgformer.py

Highlight

Results

Dataset	Original Paper	PyG
ogbn-arxiv	72.63%	72.40%

[codecov]

Codecov Report

Attention: Patch coverage is 89.74359% with 4 lines in your changes missing coverage. Please review.

Project coverage is 86.31%. Comparing base (c300f38) to head (659a081).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
torch_geometric/nn/attention/sgformer.py	89.47%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9904      +/-   ##
==========================================
- Coverage   86.56%   86.31%   -0.25%     
==========================================
  Files         490      491       +1     
  Lines       32294    32328      +34     
==========================================
- Hits        27954    27905      -49     
- Misses       4340     4423      +83     

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