docstr: Tests the finite difference function.
docstr_tok: ["Tests","the","finite","difference","function","."]
code: def test_shape_is_correct(self):
      """Tests the finite difference function."""
      n_batch = 2
      n_time = 125
      n_freq = 100
      mag = tf.ones([n_batch, n_time, n_freq])
  
      diff = core.diff
      delta_t = diff(mag, axis=1)
      self.assertEqual(delta_t.shape[1], mag.shape[1] - 1)
      delta_delta_t = diff(delta_t, axis=1)
      self.assertEqual(delta_delta_t.shape[1], mag.shape[1] - 2)
      delta_f = diff(mag, axis=2)
      self.assertEqual(delta_f.shape[2], mag.shape[2] - 1)
      delta_delta_f = diff(delta_f, axis=2)
      self.assertEqual(delta_delta_f.shape[2], mag.shape[2] - 2)
  
imports: from absl.testing import parameterized
  from ddsp import core
  import librosa
  import numpy
  from scipy import signal
  import tensorflow.compat.v2
label_desc: Use image node_id 1 for calling the DiffTest obj's underlying member method code with example usage: obj.test_shape_is_correct() without return types