* [Fix] Fix the bug that binary cross entropy loss doesn't support single channel input

* imcrease coverage

* modify implementation

* increase coverage

* add assert

* modify implementation

* enshollow condition judge

* fix

* [Fix] Add avg_non_ignore in cross entropy loss

* [Fix] Add avg_non_ignore in cross entropy loss

* add docstring

* fix ut

* fix docstring and comments

* fix

* fix bce

* fix avg_factor in BCE and add more ut

* add avg_non_ignore

* add more ut

* fix part of ut

* fix part of ut

* test avg_non_ignore would not affect ce/bce when reduction none/sum

* test avg_non_ignore would not affect ce/bce when reduction none/sum/mean

* re-organize ut

* re-organize ut

* re-organize ut

* re-organize hardcode case

* fix parts of comments

* fix another parts of comments

* fix

* [Fix] Fix the bug that when all pixels in an image is ignored, the accuracy calculation raises ZeroDivisionError

* use eps

* all close

* add ignore test

* add eps

* make accuracy take into account ignore_index

* add UT for accuracy

* [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>

* multiple losses

* fix lint error

* fix typos

* fix typos

* Adding Attribute

* Fixing loss_ prefix

* Fixing loss_ prefix

* Fixing loss_ prefix

* Add Same

* loss_name must has 'loss_' prefix

* Fix unittest

* Fix unittest

* Fix unittest

* Update mmseg/models/decode_heads/decode_head.py

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

* Add header for files

* Delete header in config files

* support reading class_weight from file in loss function

* add unit test of loss with class_weight from file

* minor fix

* move get_class_weight to utils

* refactor test organization

* fixed se layer

* update mmcv uper bound