class_docstr: Abstract two-dimensional layer that downsamples values in a kernel to a single value.
  
  Parameters
  ----------
  input_size : tuple
      the size of the input.
  output_size : tuple
      the size of the output.
  strides : matrix-like
      stride of the kernel.
  activation : str or None
      activation function name
  input_index_mapper : IndexMapper or None
      map indexes from this layer index to the input layer index size
docstr: Return the depth of the kernel
docstr_tok: ["Return","the","depth","of","the","kernel"]
code: def kernel_depth(self):
      """Return the depth of the kernel"""
      raise NotImplementedError()
  
imports: import itertools
  import numpy
label_desc: Use image node_id 4 for calling the Layer2D obj's underlying member method code with example usage: obj.kernel_depth() without return types