Adding example use case of mimic4

[kimpossible6]

1. uiuc-598 dlh group project - ziweim2, yilin17
2. What type of contribution you are making: example use case
3. demonstrates how to load the MIMIC-IV dataset, apply the Readmission30DaysMIMIC4 task, split the sample dataset by patients, and inspect a batch of features
4. replace the dir path and run