Describe the feature

Motivation
Currently, UNet backbone in mmseg could not be feed with different scale of images.
Hope to support dynamic feature.
Most related code is here

Related resources
If there is an official code release or third-party implementations, please also provide the information here, which would be very helpful.

Additional context
Add any other context or screenshots about the feature request here.
If you would like to implement the feature and create a PR, please leave a comment here and that would be much appreciated.