[ViT] Support fine-tuning with different image resolution (#5025)

* add from_checkpoint method for vit

* remove useless change

* Making interpolate_embeddings a utility function

* remove logging

* fix type hint

* fix return type check

* ad  retuurns in docsting & unify type hint

* remove useless import

* fix issue: 'type' object is not subscriptable

* Fixing typing issues

* Making interpolation mode configurable

* formatting
Co-authored-by: Vasilis Vryniotis <datumbox@users.noreply.github.com>

torchvision/prototype/models/vision_transformer.py

@@ -379,3 +379,78 @@ def vit_l_32(*, weights: Optional[ViT_L_32_Weights] = None, progress: bool = Tru
         progress=progress,
         **kwargs,
     )
+
+
+def interpolate_embeddings(
+    image_size: int,
+    patch_size: int,
+    model_state: "OrderedDict[str, torch.Tensor]",
+    interpolation_mode: str = "bicubic",
+    reset_heads: bool = False,
+) -> "OrderedDict[str, torch.Tensor]":
+    """This function helps interpolating positional embeddings during checkpoint loading,
+    especially when you want to apply a pre-trained model on images with different resolution.
+
+    Args:
+        image_size (int): Image size of the new model.
+        patch_size (int): Patch size of the new model.
+        model_state (OrderedDict[str, torch.Tensor]): State dict of the pre-trained model.
+        interpolation_mode (str): The algorithm used for upsampling. Default: bicubic.
+        reset_heads (bool): If true, not copying the state of heads. Default: False.
+
+    Returns:
+        OrderedDict[str, torch.Tensor]: A state dict which can be loaded into the new model.
+    """
+    # Shape of pos_embedding is (1, seq_length, hidden_dim)
+    pos_embedding = model_state["encoder.pos_embedding"]
+    n, seq_length, hidden_dim = pos_embedding.shape
+    if n != 1:
+        raise ValueError(f"Unexpected position embedding shape: {pos_embedding.shape}")
+
+    new_seq_length = (image_size // patch_size) ** 2 + 1
+
+    # Need to interpolate the weights for the position embedding.
+    # We do this by reshaping the positions embeddings to a 2d grid, performing
+    # an interpolation in the (h, w) space and then reshaping back to a 1d grid.
+    if new_seq_length != seq_length:
+        # The class token embedding shouldn't be interpolated so we split it up.
+        seq_length -= 1
+        new_seq_length -= 1
+        pos_embedding_token = pos_embedding[:, :1, :]
+        pos_embedding_img = pos_embedding[:, 1:, :]
+
+        # (1, seq_length, hidden_dim) -> (1, hidden_dim, seq_length)
+        pos_embedding_img = pos_embedding_img.permute(0, 2, 1)
+        seq_length_1d = int(math.sqrt(seq_length))
+        torch._assert(seq_length_1d * seq_length_1d == seq_length, "seq_length is not a perfect square!")
+
+        # (1, hidden_dim, seq_length) -> (1, hidden_dim, seq_l_1d, seq_l_1d)
+        pos_embedding_img = pos_embedding_img.reshape(1, hidden_dim, seq_length_1d, seq_length_1d)
+        new_seq_length_1d = image_size // patch_size
+
+        # Perform interpolation.
+        # (1, hidden_dim, seq_l_1d, seq_l_1d) -> (1, hidden_dim, new_seq_l_1d, new_seq_l_1d)
+        new_pos_embedding_img = nn.functional.interpolate(
+            pos_embedding_img,
+            size=new_seq_length_1d,
+            mode=interpolation_mode,
+            align_corners=True,
+        )
+
+        # (1, hidden_dim, new_seq_l_1d, new_seq_l_1d) -> (1, hidden_dim, new_seq_length)
+        new_pos_embedding_img = new_pos_embedding_img.reshape(1, hidden_dim, new_seq_length)
+
+        # (1, hidden_dim, new_seq_length) -> (1, new_seq_length, hidden_dim)
+        new_pos_embedding_img = new_pos_embedding_img.permute(0, 2, 1)
+        new_pos_embedding = torch.cat([pos_embedding_token, new_pos_embedding_img], dim=1)
+
+        model_state["encoder.pos_embedding"] = new_pos_embedding
+
+        if reset_heads:
+            model_state_copy: "OrderedDict[str, torch.Tensor]" = OrderedDict()
+            for k, v in model_state.items():
+                if not k.startswith("heads"):
+                    model_state_copy[k] = v
+            model_state = model_state_copy
+
+    return model_state