Incompatibility with TF2.0

[shkarupa-alex]

Description

Got an error when running t2t-trainer --registry_help with installed "tensorflow alpha"

Environment information

OS: MacOS X

$ pip freeze | grep tensor
mesh-tensorflow==0.0.5
tensor2tensor==1.13.1
tensorboard==1.12.0
tensorflow==2.0.0a0
tensorflow-datasets==1.0.1
tensorflow-fold==0.0.1
tensorflow-hub==0.1.1
tensorflow-metadata==0.13.0
tensorflow-serving-api==1.7.0
tensorflow-tensorboard==1.5.1
tensorflow-zero==0.0.4

$ python -V
Python 2.7.15

For bugs: reproduction and error logs

# Steps to reproduce:
pip install -U --pre tensorflow
pip install -U tensor2tensor
pip uninstall tensorflow-probability
pip install -U tfp-nightly
t2t-trainer --registry_help

# Error logs:
Traceback (most recent call last):
  File "/usr/local/bin/t2t-trainer", line 23, in <module>
    from tensor2tensor.bin import t2t_trainer
  File "/usr/local/lib/python2.7/site-packages/tensor2tensor/bin/t2t_trainer.py", line 25, in <module>
    from tensor2tensor import models  # pylint: disable=unused-import
  File "/usr/local/lib/python2.7/site-packages/tensor2tensor/models/__init__.py", line 25, in <module>
    from tensor2tensor.layers import modalities  # pylint: disable=g-import-not-at-top
  File "/usr/local/lib/python2.7/site-packages/tensor2tensor/layers/modalities.py", line 28, in <module>
    from tensor2tensor.layers import common_attention
  File "/usr/local/lib/python2.7/site-packages/tensor2tensor/layers/common_attention.py", line 5074, in <module>
    @expert_utils.add_var_scope()
  File "/usr/local/lib/python2.7/site-packages/tensor2tensor/utils/expert_utils.py", line 68, in add_var_scope
    return add_scope(scope, scope_fn=tf.variable_scope)
AttributeError: 'module' object has no attribute 'variable_scope'

[afrozenator]

Hi @shkarupa-alex -- We aren't compatible with TF 2.0 yet.

Please checkout https://github.com/tensorflow/tensor2tensor/issues/1478 as well.