Change tfe to tf

Please note that eager execution is now officially included in the main module. 
So, no need for `import tensorflow.contrib.eager as tfe`.  With that said, all occurences of `tfe` might need to be changed to `tf`, since all methods can be now accessed direclty from `tf`.

e.g. 

```tfe.GradientTape()``` --> ```tf.GradientTape()```