Add PerSAM
# What does this PR do?

This PR adds the PerSAM model.

Question: when you do:

```
from transformers import PerSamModel

model = PerSamModel.from_pretrained("facebook/sam-vit-huge")
```

you get this warning:

```
You are using a model of type sam to instantiate a model of type persam. This is not supported for all configurations of models and can yield errors.
```

was wondering whether we could suppress this warning. PerSAM uses the exact same weights as the original SAM model, just modifies the forward pass with 2 additional arguments. Currently the model_type is set to "persam" in `PerSamConfig`.


_The documentation is not available anymore as the PR was closed or merged._<|||||>Ok, will close this PR in favor of modifying `modeling_sam.py`.