class_docstr: Test for niftynet.layer.subpixel.SubPixelLayer.
  Mostly adapted from convolution_test.py
code: def get_2d_input(self, shape=(2, 16, 16, 4)):
      x_2d = tf.ones(shape)
      return x_2d
  
file_path: NiftyNet/tests/subpixel_test.py
imports: from __future__ import division, absolute_import, print_function
  import functools
  import numpy
  import tensorflow
  from niftynet.layer.subpixel import SubPixelLayer
  from tests.niftynet_testcase import NiftyNetTestCase
label_desc: Use image node_id 2 for calling the SubPixelTest obj's underlying member method code with example usage: obj.get_2d_input(shape) and returns: x_2d