* Add support for prim::If and prim::Loop with test cases

* rebase and fix tests

* add some comments

* simplifying, fix float cast

* parse -> convert

* recursivly retrive ops in get_all_op_names

* use multiple return values from block correctly, simplify loop convert

* choose dtype properly for zeros and ones

* simplifying, replace convert_inputs with _get_relay_input_vars

* fix for while loop with non input dependent init cond

* add assert on loop var update

* move the condition around

* better testing for seg models

* rebase fix, disable inception v3 in quant test as it is too slow to
load with torch-1.4 + torchvision 0.5

* simplify and add more comparison op converter