Add tutorial for multiple instance learning (MIL) code for 2D pathology WSI classification (ETA 11/12/2021)

[myron]

Add implementation of "Accounting for Dependencies in Deep Learning based Multiple Instance Learning for Whole Slide Imaging"

Input format: 2D TIFF (WSI)
Output format: class label

[myron]

Preliminary draft PR were added to tutorials. working on moving some components into monai core
Project-MONAI/tutorials#431

[wyli]

addressed by Project-MONAI/tutorials#431