message: Add a comment to tf2.py about what does TF2_BEHAVIOR=1 enable.
diff: @@ -39,6 +39,9 @@ def enable_v2_behavior():
  This function is called in the main TensorFlow `__init__.py` file, user should
  not need to call it, except during complex migrations.
  """
  + # TF2 behavior is enabled if either 1) enable_v2_behavior() is called or
  + # 2) the TF2_BEHAVIOR=1 environment variable is set. In the latter case,
  + # the modules below independently check if tf2.enabled().
  tf2.enable()
  ops.enable_eager_execution()
  tensor_shape.enable_v2_tensorshape() # Also switched by tf2
  