[CS598] Add HGNN model

[bwchiu]

1. Who you are (netid if Illinois student) brianc6

2. What type of contribution you are making Model contribution

3. A high level description of what you've done Implemented a Hypergraph Neural Network (HGNN) that combines hypergraph convolution with temporal dynamics. Uses GRU-based temporal aggregation with attention. Per-feature processing handles variable-length sequences.

4. A quick rundown of what files to look at to test the implementation Model: pyhealth/models/hgnn.py Tests: tests/core/test_hgnn.py Run model: python pyhealth/models/hgnn.py Run tests: python tests/core/test_hgnn.py