Add option to save the result images of running tools/test (#2414)

* Use apis/inference in detectors show_result

* Add images_out_dir arg to test apis

* Include --images_out_dir in assertion

* Fix single class parsing

* Remove single class hack

* Move import

* Add example of saving results

* Fix list number

* Refactor show_result

* Update docs

* Update __init__

* Fix CLASSES reference

* Remove unnecessar assert for custom classes

* Use - instead of _

* Use - instead of _

* Rename images_out_dir -> out_dir

* Remove unnecessary model.module

* Update show_result with all params and docstring

* Fix missing comma

* Set rescale always to True

* Remove outdated restriction from docs

* Drop pathlib

* Rename out_dir -> show_dir

* Update docstring

* More explicit code. Fix out_dir

* Update docstrings

* Flake8

Co-authored-by: mmeendez8 <miguelmndez@gmail.com>
Co-authored-by: sbugallo <sbugallo@gradiant.org>