Fix SETR export bug

Author: nepeplwu

paddleseg/models/backbones/vision_transformer.py

@@ -103,7 +103,8 @@ def __init__(self,
         self.proj_drop = nn.Dropout(proj_drop)
 
     def forward(self, x):
-        N, C = x.shape[1:]
+        x_shape = paddle.shape(x)
+        N, C = x_shape[1], x_shape[2]
         qkv = self.qkv(x).reshape((-1, N, 3, self.num_heads,
                                    C // self.num_heads)).transpose((2, 0, 3, 1,
                                                                     4))