[Fix] Fix the bug in binary_cross_entropy (#1527)

* [Fix] Fix the bug in binary_cross_entropy

 Fix the bug in binary_cross_entropy
'label.max() <= 1' should mask out ignore_index, since the ignore_index often set as 255.

* [Fix] Fix the bug in binary_cross_entropy, add comments

As the ignore_index often set as 255, so the binary class label check should mask out ignore_index.

Co-authored-by: Miao Zheng <76149310+MeowZheng@users.noreply.github.com>

* [Fix] Fix the bug in binary_cross_entropy

As the ignore_index often set as 255, so the binary class label check should mask out ignore_index.

Co-authored-by: Miao Zheng <76149310+MeowZheng@users.noreply.github.com>
Co-authored-by: MeowZheng <meowzheng@outlook.com>