Rename semantic segmentation outputs
# What does this PR do?

This PR renames `SemanticSegmentationModelOutput` to `SemanticSegmentationOutput`, because of simplicity.

We also have `SequenceClassifierOutput` and `TokenClassifierOutput` for instance, so the "model" in the name is not really useful.
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15849). All of your documentation changes will be reflected on that endpoint.<|||||>`Classifier` is a model name, whereas `Segmentation` is not. We could go for `SemanticSegmenter` if you dislike `SemanticSegmentationModelOutput`, but `SemanticSegmentationOutput` does not match the other model output names.<|||||>Agree with @sgugger , similarly to `SemanticSegmentationModelOutput`, I've added a `ImageClassificationModelOutput` (in resnet pr for now) following the same naming convention<|||||>Should we resurrect this? We went for `ImageClassifierOutput`, maybe we can go for `SemanticSegmenterOutput` as @sgugger proposed<|||||>@FrancescoSaverioZuppichini updated the PR accordingly