Make Swin work with VisionEncoderDecoderModel (#15527)

* Add attribute_map

* Add mention in docs

* Set hidden_size attribute correctly

* Add note about Transformer-based models only
Co-authored-by: Niels Rogge <nielsrogge@Nielss-MBP.localdomain>

docs/source/model_doc/vision-encoder-decoder.mdx

@@ -13,8 +13,8 @@ specific language governing permissions and limitations under the License.
 # Vision Encoder Decoder Models
 
 The [`VisionEncoderDecoderModel`] can be used to initialize an image-to-text-sequence model with any
-pretrained vision autoencoding model as the encoder (*e.g.* [ViT](vit), [BEiT](beit), [DeiT](deit))
-and any pretrained language model as the decoder (*e.g.* [RoBERTa](roberta), [GPT2](gpt2), [BERT](bert)).
+pretrained Transformer-based vision autoencoding model as the encoder (*e.g.* [ViT](vit), [BEiT](beit), [DeiT](deit), [Swin](swin))
+and any pretrained language model as the decoder (*e.g.* [RoBERTa](roberta), [GPT2](gpt2), [BERT](bert), [DistilBERT](distilbert)).
 
 The effectiveness of initializing image-to-text-sequence models with pretrained checkpoints has been shown in (for
 example) [TrOCR: Transformer-based Optical Character Recognition with Pre-trained Models](https://arxiv.org/abs/2109.10282) by Minghao Li, Tengchao Lv, Lei Cui, Yijuan Lu, Dinei Florencio, Cha Zhang,

src/transformers/models/swin/configuration_swin.py

@@ -90,6 +90,10 @@ class SwinConfig(PretrainedConfig):
     ```"""
     model_type = "swin"
 
+    attribute_map = {
+        "num_attention_heads": "num_heads",
+    }
+
     def __init__(
         self,
         image_size=224,
@@ -130,3 +134,6 @@ class SwinConfig(PretrainedConfig):
         self.path_norm = patch_norm
         self.layer_norm_eps = layer_norm_eps
         self.initializer_range = initializer_range
+        # we set the hidden_size attribute in order to make Swin work with VisionEncoderDecoderModel
+        # this indicates the channel dimension after the last stage of the model
+        self.hidden_size = embed_dim * 8