Fix DeepLabV3Plus encoder depth (#986)

* fix issue #377

* modify docstring for upsampling of DeepLabV3Plus

* modify type hint and value check

segmentation_models_pytorch/decoders/deeplabv3/decoder.py

@@ -70,15 +70,20 @@ def forward(self, *features):
 class DeepLabV3PlusDecoder(nn.Module):
     def __init__(
         self,
-        encoder_channels: Sequence[int, ...],
+        encoder_channels: Sequence[int],
+        encoder_depth: Literal[3, 4, 5],
         out_channels: int,
         atrous_rates: Iterable[int],
         output_stride: Literal[8, 16],
         aspp_separable: bool,
         aspp_dropout: float,
     ):
         super().__init__()
-        if output_stride not in {8, 16}:
+        if encoder_depth not in (3, 4, 5):
+            raise ValueError(
+                "Encoder depth should be 3, 4 or 5, got {}.".format(encoder_depth)
+            )
+        if output_stride not in (8, 16):
             raise ValueError(
                 "Output stride should be 8 or 16, got {}.".format(output_stride)
             )
@@ -104,7 +109,14 @@ def __init__(
         scale_factor = 2 if output_stride == 8 else 4
         self.up = nn.UpsamplingBilinear2d(scale_factor=scale_factor)
 
-        highres_in_channels = encoder_channels[-4]
+        if encoder_depth == 3 and output_stride == 8:
+            self.highres_input_index = -2
+        elif encoder_depth == 3 or encoder_depth == 4:
+            self.highres_input_index = -3
+        else:
+            self.highres_input_index = -4
+
+        highres_in_channels = encoder_channels[self.highres_input_index]
         highres_out_channels = 48  # proposed by authors of paper
         self.block1 = nn.Sequential(
             nn.Conv2d(
@@ -128,7 +140,7 @@ def __init__(
     def forward(self, *features):
         aspp_features = self.aspp(features[-1])
         aspp_features = self.up(aspp_features)
-        high_res_features = self.block1(features[-4])
+        high_res_features = self.block1(features[self.highres_input_index])
         concat_features = torch.cat([aspp_features, high_res_features], dim=1)
         fused_features = self.block2(concat_features)
         return fused_features

segmentation_models_pytorch/decoders/deeplabv3/model.py

@@ -129,7 +129,8 @@ class DeepLabV3Plus(SegmentationModel):
             Available options are **"sigmoid"**, **"softmax"**, **"logsoftmax"**, **"tanh"**, **"identity"**,
                 **callable** and **None**.
             Default is **None**
-        upsampling: Final upsampling factor. Default is 4 to preserve input-output spatial shape identity
+        upsampling: Final upsampling factor. Default is 4 to preserve input-output spatial shape identity. In case
+            **encoder_depth** and **encoder_output_stride** are 3 and 16 resp., set **upsampling** to 2 to preserve.
         aux_params: Dictionary with parameters of the auxiliary output (classification head). Auxiliary output is build
             on top of encoder if **aux_params** is not **None** (default). Supported params:
                 - classes (int): A number of classes
@@ -150,7 +151,7 @@ class DeepLabV3Plus(SegmentationModel):
     def __init__(
         self,
         encoder_name: str = "resnet34",
-        encoder_depth: int = 5,
+        encoder_depth: Literal[3, 4, 5] = 5,
         encoder_weights: Optional[str] = "imagenet",
         encoder_output_stride: Literal[8, 16] = 16,
         decoder_channels: int = 256,
@@ -177,6 +178,7 @@ def __init__(
 
         self.decoder = DeepLabV3PlusDecoder(
             encoder_channels=self.encoder.out_channels,
+            encoder_depth=encoder_depth,
             out_channels=decoder_channels,
             atrous_rates=decoder_atrous_rates,
             output_stride=encoder_output_stride,