[Feature] Support multiple losses during training (#818)

* 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>