Add semantic script, trainer
# What does this PR do?

This PR adds `run_semantic_segmentation.py`, which leverages the Trainer API for fine-tuning any model supported by `AutoModelForSemanticSegmentation`.

To do:

- [x] fix WandB logging, which doesn't seem to like the fact that metrics are turned into lists. Might need some help here from @sgugger.
- [x] also wondering why the [model card](https://huggingface.co/nielsr/segformer-finetuned-sidewalk-trainer) doesn't include the dataset name (even though it has a default value in the script):

<img width="535" alt="Screenshot 2022-04-19 at 16 19 22" src="https://user-images.githubusercontent.com/48327001/164025421-2c74a31e-26b9-4652-a99a-ddedbd54428b.png">
_The documentation is not available anymore as the PR was closed or merged._