Update simmim_neck.py

Update SimMIMLinearDecoder with `target_channels`. The downstream loss for SimMIM i.e. the `PixelReconstructionLoss` already allows user to set the number of channels through the `channel` argument. 
Useful in cases when reconstructing non-rgb images.

mmpretrain/models/necks/simmim_neck.py

@@ -15,14 +15,15 @@ class SimMIMLinearDecoder(BaseModule):
     Args:
         in_channels (int): Channel dimension of the feature map.
         encoder_stride (int): The total stride of the encoder.
+        target_channels (int): Channel dimensions of original image.
     """
 
-    def __init__(self, in_channels: int, encoder_stride: int) -> None:
+    def __init__(self, in_channels: int, encoder_stride: int, target_channels: int = 3) -> None:
         super().__init__()
         self.decoder = nn.Sequential(
             nn.Conv2d(
                 in_channels=in_channels,
-                out_channels=encoder_stride**2 * 3,
+                out_channels=encoder_stride**2 * target_channels,
                 kernel_size=1),
             nn.PixelShuffle(encoder_stride),
         )