6765 update GeneralizedDiceLoss

[wyli]

Fixes #6765

Description

as discussed in #6765, when batch=True the loss should still return 1 aggregated value instead of C channels. #5466 is not actually achievable with this formulation.

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.

[KumoLiu]

Thanks for the update, LGTM.

[wyli]

/build