[Fix] Add avg_non_ignore in cross entropy loss (#1409)

* [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