Migrate examples and documentation to TF core

[amygdala]

Is there some reason these examples are still using tf.contrib.learn?
e.g. https://github.com/tensorflow/transform/blob/master/examples/census_example.py#L31

(seems like that's not going to help demystify tft for users..)

Update: similarly re: mentions of "tf.Learn" in the docs, e.g. here: https://github.com/tensorflow/transform/blob/master/getting_started.md
I think this will just confuse people.

[elmer-garduno]

Thanks @amygdala great point, I'll update the issue name and we'll work on migrating the examples to TF core.

[amygdala]

oh great, thanks!

[amygdala]

Update: okay, I might just put in a PR for this instead... as I mentioned to Elmer, I did the conversion for the sentiment one for my own purposes and it would be straightforward to just do the rest of it.

[elmer-garduno]

Thanks Amy, a PR will be very welcome.

[KesterTong]

Examples are now using core