Make the flexible unet more extensible to different backbones

[binliunls]

Signed-off-by: binliu [email protected]

Fixes #5011 .

Description

Currently, the FlexibleUNet structure only supports the efficient-net series as the backbone and it is hard to extend to other network structures. A more extensible and convenient way to add more benchmark or user defined backbones will make it more flexible. I plan to make it by doing steps below

1. A base backbone/encoder class that defines interfaces like the number of output feature maps, a list of output feature map channels, string names of backbones and so on.

2. A register who can dynamically regist backbones to a dict that will be used during the flexible unet initialization.

Status

Work in progress

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.