docstr: Mock the init logic for read_mask_pil function
  
  Instead of reading a mask at location specified by `path`, a PIL mask image of
  size (20, 40) is returned.
code: def read_mask_pil(path: str) -> Optional[Image.Image]:
      """Mock the init logic for read_mask_pil function
  
      Instead of reading a mask at location specified by `path`, a PIL mask image of
      size (20, 40) is returned.
      """
      return Image.new("L", (20, 30))
  
file_path: ml-cvnets/tests/data/datasets/segmentation/mock_ade20k.py
imports: import argparse
  from typing import Optional
  from PIL import Image
  from data.datasets.segmentation.ade20k import ADE20KDataset
label_desc: Use image node_id 3 for calling the MockADE20KDataset obj's underlying member method code with example usage: obj.read_mask_pil(path) and returns: Image