Add GIT-Mol

[xnuohz]

Issue

• [Community Sprint] GNNs<>LLMs 🚀  #9694
• GIT-Mol: Dataset+Model+Unit tests+Example #9700

Feature Summary

• Add GitMolDataset
• Add GITMol as GNN & LLM Co-training model to PyG
• Add an example for pre-training
• Limited hardware resources, so the full training pipeline was not tested
• Multi modal cross attention shares the same weight, not aligned with the original paper

[xnuohz]

@puririshi98 Local CI passed.