Add/mm retain adacare

[jhnwu3]

This pull request introduces new example scripts and documentation updates to support multimodal and hyperparameter-tuned drug recommendation models on the MIMIC-IV dataset, specifically using AdaCare and RETAIN model variants. It also improves the usability and clarity of existing example scripts.

Major additions and improvements:

1. New example scripts for drug recommendation:

• Added drug_recommendation_mimic4_adacare.py demonstrating how to use the AdaCare model for drug recommendation on MIMIC-IV, including data loading, task setup, training, evaluation, and prediction inspection.
• Added drug_recommendation_mimic4_adacare_optuna.py, providing an end-to-end example of hyperparameter tuning for AdaCare using Optuna, covering parameter search, training, and evaluation.
• Added drug_recommendation_mimic4_multimodal_retain.py, showcasing the use of the new MultimodalRETAIN model for handling both sequential and non-sequential EHR features, with detailed steps and comparison to vanilla RETAIN.

2. Documentation enhancements:

• Updated AdaCare and RETAIN model documentation to include the new MultimodalAdaCare and MultimodalRETAIN classes, improving discoverability and API reference completeness. [1] [2]

3. Usability and bug fixes in existing examples:

• Ensured all example scripts use a consistent dataset cache directory for reproducibility and efficiency.
• Fixed sample access in the RETAIN example to use the correct dataset indexing method, improving code reliability.

[jhnwu3]

Need help looking into file read checks like this. Mainly, for when the agent spins up multiple caching jobs at once, which leads to downstream issues like overwriting files, etc.