add weights and conversion script for "DeepLab V3"

[sachinprasadhs]

TODO:

• [ ] add weights conversion script here - https://github.com/keras-team/keras-hub/tree/master/tools/checkpoint_conversion
• [ ] Verify output with real inputs
• [ ] provide a working colab demo

Example PR - https://github.com/keras-team/keras-hub/pull/1855.