Merge target into head (#2429)

* merge target into head

* remove squeeze in image_to_level, add doc string

* reformat doc string

* update docs, rename to get_target

* reanme unify all target functions to get_targets

* set test_cfg as default cfg for get_bboxes

* fixed test

* rename get_targets in atss, fcos ,fovea, reppoint

* rename get_det_bboxes to get_bboxes