[Feature] Add focal loss (#1024)

* [Feature] add focal loss

* fix the bug of 'non' reduction type

* refine the implementation

* add class_weight and ignore_index; support different alpha values for different classes

* fixed some bugs

* fix bugs

* add comments

* modify test

* Update mmseg/models/losses/focal_loss.py

Co-authored-by: Junjun2016 <hejunjun@sjtu.edu.cn>

* update test_focal_loss.py

* modified the implementation

* Update mmseg/models/losses/focal_loss.py

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

* update focal_loss.py

Co-authored-by: Junjun2016 <hejunjun@sjtu.edu.cn>
Co-authored-by: Jerry Jiarui XU <xvjiarui0826@gmail.com>