Add MIMIC-III readmission Transformer example and tests

[RishabhRanganathan2004]

Add MIMIC-III Readmission Prediction Example Using Transformer + Tests

Summary

This PR adds a new example demonstrating 30-day readmission prediction on MIMIC-III using the Transformer model. While PyHealth includes Transformer examples and readmission examples, this is the first to combine Transformer + readmission + MIMIC-III, filling a clear gap in the existing examples.

What's Included

New Example

examples/mimic3_readmission_transformer.py

• Full 5-stage PyHealth pipeline
• Supports Synthetic MIMIC-III and local MIMIC-III
• Well-documented module-level docstring
• CLI arguments for reproducible runs

New Tests

tests/test_mimic3_readmission_transformer.py

• Lightweight integration tests
• Validates the example pipeline end-to-end
• Skips automatically unless local or approved remote data is available

Why This Is Useful

• Introduces a missing model–task combination to the example library
• Demonstrates how to run Transformer on clinical prediction tasks
• Improves reproducibility of PyHealth examples
• Adds test coverage to ensure long-term stability

Testing

Synthetic MIMIC-III (online):

export PYHEALTH_RUN_ONLINE_EXAMPLES=1
pytest -q