class_docstr: Two-dimensional pooling layer.
  
  Parameters
  ----------
  input_size : tuple
      the size of the input.
  output_size : tuple
      the size of the output.
  strides : matrix-like
      stride of the kernel.
  pool_func : str
      name of function used to pool values in a kernel to a single value.
  transpose : bool
      True iff input matrix is accepted in transposed (i.e. column-major)
      form.
  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"""
      return self._kernel_depth
  
imports: import itertools
  import numpy
label_desc: Use image node_id 3 for calling the PoolingLayer2D obj's underlying member method code with example usage: obj.kernel_depth() and returns: self