Add GP2F model

[Marchtlp]

Description

Add the GP2F model and a Cora-to-PubMed 1-shot reproduction example.

The implementation includes:

• GP2FModel and the GRACE pretraining components in gammagl/models/gp2f.py
• Cora GRACE pretraining trainer
• Memory-safe PubMed 1-shot GP2F trainer
• Reproduction instructions and expected results in examples/gp2f/README.md

Checklist

• The PR title starts with [Model]
• Changes are complete (i.e. I finished coding on this PR)
• Code is well-documented
• To the best of my knowledge, examples are either not affected by this change,
  or have been fixed to be compatible with this change

Changes

• Add the GP2F model implementation and public model export
• Add Cora GRACE pretraining and PubMed 1-shot transfer examples
• Add reproduction documentation