Segresnet2

[myron]

This adds improvements to SegResNet semantic segmentation to include deep supervision feature (among other things).

I've decided to add a new network named SegResnetDS instead of replacing the existing SegResNet, because it would have broken all previously pretrained model checkpoints.

Please take a look, and if it looks good, I can add unit tests. All the helper functions, and blocks are inside of the same file (which I prefer, since it's easier to read) instead of spreading it among other files (e.g segresnet_ds_block or segresnet_utils).

Description

A few sentences describing the changes proposed in this pull request.

Types of changes

• [x] 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.