Revisit "include_background" variable name in loss functions

I believe that using "include_channel_zero" as an alternative to "include_background" may not be the best choice due to the loss of argument semantics. In contrast, the name "include_background" directly conveys the purpose of this argument to the user. The core issue of original post is that the channel of background is confusing. One potential solution could be to introduce an optional argument, such as background_channel: int | Sequence[int] = 0, to clarify this aspect. BTW, argument such as include_channel: int | Sequence[int] = 0 could be better than "include_channel_zero" .

Originally posted by @ChenglongWang in https://github.com/Project-MONAI/MONAI/discussions/6915#discussioncomment-7097583

See whole discussion for further feedback.

Read the linked discussion and its feedback first, then inspect the loss functions affected by the include_background naming question. The work is complete only after the project has settled on a clear argument name and background-channel representation, with the affected API consistently reflecting that decision.