* Move type prop tests to separate file, add cases with exceptions thrown
* Add visitor test, add step to pbc attributes
* SLT for CPU, adjust GPU and SSLT to serve the step attr
* Add backend test
* Fix variance issue, verify by SLT
* Formatting fix
* Add another backend testcase
* Add PriorBoxClustered to trusted ops list
* Add step attr description, remove obsolete comments from ref impl
* Test multiple step values in SLT
Bartosz Lesniewski
c14edd4134