Fixed misguiding weight mode documentation

[simojens]

Description

DeepSupervisionLoss has a small typo in the documentation for 'exp' weight mode. The correct weights for this mode should be starting at 1, not 0 and then decrease by power of two. Might be worth fixing as it made me check the source code.

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.

[ericspod]

Good spot thanks!

[KumoLiu]

/build