added mimic3 llm diagnosis task and test

[stephenmoy14]

Stephen Moy moy26 N/A Paper Title N/A Paper Link This task aggregates MIMIC-III clinical notes by patient and applies a large language model (e.g., FLAN-T5) to classify whether the patient has a specified diagnosis. It integrates preprocessing, patient-level aggregation, and LLM-based classification into a single PyHealth Task class for reproducibility and open-source contribution.