add conv mixer (#4280)

ppdet/modeling/layers.py

@@ -1388,3 +1388,37 @@ class MultiHeadAttention(nn.Layer):
         if self.need_weights:
             outs.append(weights)
         return out if len(outs) == 1 else tuple(outs)
+
+
+@register
+class ConvMixer(nn.Layer):
+    def __init__(
+            self,
+            dim,
+            depth,
+            kernel_size=3, ):
+        super().__init__()
+        self.dim = dim
+        self.depth = depth
+        self.kernel_size = kernel_size
+
+        self.mixer = self.conv_mixer(dim, depth, kernel_size)
+
+    def forward(self, x):
+        return self.mixer(x)
+
+    @staticmethod
+    def conv_mixer(
+            dim,
+            depth,
+            kernel_size, ):
+        Seq, ActBn = nn.Sequential, lambda x: Seq(x, nn.GELU(), nn.BatchNorm2D(dim))
+        Residual = type('Residual', (Seq, ),
+                        {'forward': lambda self, x: self[0](x) + x})
+        return Seq(*[
+            Seq(Residual(
+                ActBn(
+                    nn.Conv2D(
+                        dim, dim, kernel_size, groups=dim, padding="same"))),
+                ActBn(nn.Conv2D(dim, dim, 1))) for i in range(depth)
+        ])