Shift from tf.compat to proper tensorflow 2

[mhuen]

The code base currently utilizes tf.compat.* symbols. This should be updated to use more idomatic tensorflow 2 logic. This will require considerable rewriting of some of the core components. It is thus unlikely that backwards compatibility for existing models is possible. Therefore, this change should be implemented in a future version 2 of dnn_reco together with other necessary, but potentially breaking changes.