class_docstr: Fill the background by a constant 
class_docstr_tok: ["Fill","the","background","by","a","constant"]
code: def _fill(self, background_shape, img):
      assert img.ndim in [3, 2]
      if img.ndim == 3:
          return_shape = background_shape + (img.shape[2],)
      else:
          return_shape = background_shape
      return np.zeros(return_shape, dtype=img.dtype) + self.value
  
imports: import numpy
  from abc import abstractmethod
  from .base import ImageAugmentor
  from .transform import TransformFactory
label_desc: Use image node_id 2 for calling the ConstantBackgroundFiller obj's underlying member method code with example usage: obj._fill(background_shape, img) and returns: unknown