Add `TF_MODEL_FOR_SEMANTIC_SEGMENTATION_MAPPING`
# What does this PR do?

The original goal is to fix `TFSegformerModelTest.test_keras_fit`, but it ends up the following

- Add `TF_MODEL_FOR_SEMANTIC_SEGMENTATION_MAPPING` to some `__init__` files.
- Add `training` arguments in a few layers for `TFSegformerModel`
- Update `_prepare_for_class` to deal with 2 more image tasks
- Fix `TFData2VecVisionForSemanticSegmentation` loss: we need batch dimension (without this, `test_dataset_conversion` failed - this was previously skipped due to the lack of labels)


_The documentation is not available anymore as the PR was closed or merged._<|||||>Test failures are `ValueError: Connection error` - irrelevant.<|||||>Thank you, @ydshieh for this. I appreciate the help. 