class_docstr: Extension of `MaskFile` for VOC masks.
  Removes the color pallete and optionally drops void pixels.
  
  # Arguments
      drop_void (bool): drops the void pixels, which should have the value 255.
      filepath: Path to the mask image file.
code: def to_mask(self, h, w) -> MaskArray:
      mask_arr = np.array(
          open_img(self.filepath, ensure_no_data_convert=True)
      )
      obj_ids = np.unique(mask_arr)[1:]
      masks = mask_arr == obj_ids[:, None, None]
  
      if self.drop_void:
          masks = masks[:-1, ...]
  
      return MaskArray(masks)
  
imports: from icevision.imports import *
  from icevision.utils import *
  from PIL import Image
label_desc: Use image node_id 2 for calling the VocMaskFile obj's underlying member method code with example usage: obj.to_mask(h, w) and returns: MaskArray