support transformer backbone (#465)

* vit backbone

* fix lint

* add docstrings and fix pretrained pos_embed dim not match prob

* add unittest for vit

* fix lint

* add vit based fcn configs

* fix import error

* support multiple resolution input images

* upsample pos_embed at init_weights

* support resize pos_embed at evaluation

* fix training errors

* add more unitest code for vit backbone

* unitest for uncovered code

* add norm_eval unittest

* refactor _pos_embeding

* minor change

* change var name

* rafactor init_weight

* load weights after resize

* ignore 'module' in pretrain checkpoint

* add with_cp

* add with_cp

Co-authored-by: Jiarui XU <xvjiarui0826@gmail.com>