Support transformer-style tu- encoders in UnetPlusPlus and Linknet

segmentation_models_pytorch/decoders/linknet/decoder.py

@@ -50,7 +50,7 @@ def forward(
         self, x: torch.Tensor, skip: Optional[torch.Tensor] = None
     ) -> torch.Tensor:
         x = self.block(x)
-        if skip is not None:
+        if skip is not None and skip.shape[1] != 0:
             x = x + skip
         return x
 
@@ -71,6 +71,12 @@ def __init__(
         encoder_channels = encoder_channels[::-1]
 
         channels = list(encoder_channels) + [prefinal_channels]
+        for i in range(1, len(channels) - 1):
+            # Transformer-style encoders may expose a 0-channel placeholder for the
+            # missing 1/2-scale skip. Keep the decoder stream non-empty and just
+            # skip feature fusion at that stage.
+            if channels[i] == 0:
+                channels[i] = channels[i - 1]
 
         self.blocks = nn.ModuleList(
             [

segmentation_models_pytorch/decoders/unetplusplus/decoder.py

@@ -18,6 +18,17 @@ def __init__(
         interpolation_mode: str = "nearest",
     ):
         super().__init__()
+        self.out_channels = out_channels
+        self.is_empty = out_channels == 0
+        self.interpolation_mode = interpolation_mode
+
+        if self.is_empty:
+            self.conv1 = None
+            self.attention1 = None
+            self.conv2 = None
+            self.attention2 = None
+            return
+
         self.conv1 = md.Conv2dReLU(
             in_channels + skip_channels,
             out_channels,
@@ -36,12 +47,14 @@ def __init__(
             use_norm=use_norm,
         )
         self.attention2 = md.Attention(attention_type, in_channels=out_channels)
-        self.interpolation_mode = interpolation_mode
 
     def forward(
         self, x: torch.Tensor, skip: Optional[torch.Tensor] = None
     ) -> torch.Tensor:
         x = F.interpolate(x, scale_factor=2.0, mode=self.interpolation_mode)
+        if self.is_empty:
+            height, width = x.shape[2:]
+            return x.new_empty(x.shape[0], 0, height, width)
         if skip is not None:
             x = torch.cat([x, skip], dim=1)
             x = self.attention1(x)

tests/models/test_linknet.py

@@ -1,6 +1,18 @@
+import torch
+
+import segmentation_models_pytorch as smp
+
 from tests.models import base
 
 
 class TestLinknetModel(base.BaseModelTester):
     test_model_type = "linknet"
     files_for_diff = [r"decoders/linknet/", r"base/"]
+
+    def test_timm_transformer_style_encoder(self):
+        model = smp.Linknet("tu-convnext_atto", encoder_weights=None).eval()
+
+        with torch.inference_mode():
+            output = model(torch.rand(1, 3, 256, 256))
+
+        assert output.shape == (1, 1, 256, 256)

tests/models/test_unetplusplus.py

@@ -1,4 +1,5 @@
 import segmentation_models_pytorch as smp
+import torch
 
 from tests.models import base
 
@@ -7,6 +8,14 @@ class TestUnetPlusPlusModel(base.BaseModelTester):
     test_model_type = "unetplusplus"
     files_for_diff = [r"decoders/unetplusplus/", r"base/"]
 
+    def test_timm_transformer_style_encoder(self):
+        model = smp.UnetPlusPlus("tu-convnext_atto", encoder_weights=None).eval()
+
+        with torch.inference_mode():
+            output = model(torch.rand(1, 3, 256, 256))
+
+        assert output.shape == (1, 1, 256, 256)
+
     def test_interpolation(self):
         # test bilinear
         model_1 = smp.create_model(