Make is_thing_map in Feature Extractor post_process_panoptic_segmentation defaults to all instances
# What does this PR do?

Following a discussion with @Narsil , argument `is_thing_map` in `FeatureExtractor.post_process_panoptic_segmentation` will default to consider all instances `thing`. Thus, it won't perform instances merging. 

The user should always provide a correct `is_thing_map` if he wants to merge instances, currently, the code will default to `COCO` that may be unwanted. 
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15954). All of your documentation changes will be reflected on that endpoint.<|||||>> Not particularly fond of the names `thing` vs `stuff` but I imagine this is the standards ?

More or less :) 
<|||||>Thanks @Narsil  for the very nice feedback. Following your amazing set of examples, I believe `class_ids_to_fuse` (or `label_ids_to_fuse`) is the most descriptive name we can use. 

As you said, a `Dict` is not necessary. My implementation followed more or less what was done by the authors, but we can improve it. 

I'll update the code.