Padding queue support for tf.estimator generator input pipeline (#10732)

* add padding queues support for tf.estimator generator input pipeline

* simplified version

* now with six zip

* add first docs and test

* typos and code style fix

* updated test: bug found

* now with specified padding value

* non numberic tests added

* pylint

* typo&!

* still hate typos

* additional exceptions

* indentation fix

* generator io updated for new feeding functions

* test updated, numpy corrected

* numpy error fix 2