GitLab

* Exposing LevelMapper params in MultiScaleRoIAlign

Ability to set the `canonical_scale` and `canonical_level` of LevelMapper from the init method of MultiScaleRoIAlign. 
Not sure if this is the right place to expose those parameters. 
Also, additional tests should be written.

Discussion in issue #3129

* MultiScaleRoIAlign keyword-only parameters + docs (pytorch#3129)

* Linter fix (trailing whitespace)
Co-authored-by: Francisco Massa <fvsmassa@gmail.com>