message: Update examples in docstring to use TF 2.x code
  The examples in docstrings of two APIs, tf.histogram_fixed_width_bins
  and tf.histogram_fixed_width still used TF 1.x code.
  This PR updates the docstring to use TF 2.x code in examples.
diff: @@ -68,10 +68,8 @@ def histogram_fixed_width_bins(values,
  value_range = [0.0, 5.0]
  new_values = [-1.0, 0.0, 1.5, 2.0, 5.0, 15]
  - with tf.compat.v1.get_default_session() as sess:
  indices = tf.histogram_fixed_width_bins(new_values, value_range, nbins=5)
  - variables.global_variables_initializer().run()
  - sess.run(indices) # [0, 0, 1, 2, 4, 4]
  + indices # [0, 0, 1, 2, 4, 4]
  ```
  """
  with ops.name_scope(name, 'histogram_fixed_width_bins',
  @@ -137,10 +135,8 @@ def histogram_fixed_width(values,
  value_range = [0.0, 5.0]
  new_values = [-1.0, 0.0, 1.5, 2.0, 5.0, 15]
  - with tf.compat.v1.get_default_session() as sess:
  hist = tf.histogram_fixed_width(new_values, value_range, nbins=5)
  - variables.global_variables_initializer().run()
  - sess.run(hist) => [2, 1, 1, 0, 2]
  + hist # [2, 1, 1, 0, 2]
  ```
  """
  with ops.name_scope(name, 'histogram_fixed_width',
  