Add eICU LLM synthetic mortality example

[mohsinposts]

Who I am:
Mohsin Shah (NetID: mohsins2)

Contribution type:
New example / use case of PyHealth.

High-level description:
This PR adds an example script examples/eicu_llm_synthetic_mortality.py which demonstrates how to use PyHealth with small tabular EHR-style data and LLM-generated synthetic cohorts from my CS598 project.
The example uses 10 hand-crafted ICU features and compares three training regimes for ICU mortality prediction:

Real train → Real test

GPT baseline synthetic train → Real test

GPT privacy-aware synthetic train → Real test

How to run / what files to look at

Example script: examples/eicu_llm_synthetic_mortality.py

The script directly loads the CSVs from my project repo via raw GitHub URLs:
https://github.com/mohsinposts/CS598-DLH-LLM-eICU
(real_icu_10feat.csv, synthetic_baseline_10feat_clean.csv, synthetic_privacy_10feat_clean.csv)

From the PyHealth repo root, run:

python examples/eicu_llm_synthetic_mortality.py

This prints ROC-AUC, PR-AUC, accuracy, F1, and loss.