Add Polynormer model and example

[xnuohz]

Issue

• From Graph Transformer Enhancement #9751

Usage

python examples/polynormer.py --post_bn

Highlight

Result

Dataset	Original Paper	PyG
ogbn-arxiv	73.46%	73.34%

[codecov]

Codecov Report

Attention: Patch coverage is 80.43478% with 9 lines in your changes missing coverage. Please review.

Project coverage is 86.30%. Comparing base (c300f38) to head (96ffe62).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
torch_geometric/nn/attention/polynormer.py	80.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9908      +/-   ##
==========================================
- Coverage   86.56%   86.30%   -0.26%     
==========================================
  Files         490      491       +1     
  Lines       32294    32335      +41     
==========================================
- Hits        27954    27907      -47     
- Misses       4340     4428      +88     

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