make "smart_cond" api public and reusable (#13954)

* make smart_cond api public and reusable

* adjust function comment space

* change `smart_cond` naming schema consistent with `cond`

* Move `smart_cond` and `smart_constant_value` to tf.contrib.framework

* resolve loop import

* remove redundant constant_value

* fix function name typo

* Fix function parameter name usage

* Fix function parameter name usage(2)

* To support 0/1 interger pred

* check integer value after type check

* Lint fixes