fix attn export

ppocr/modeling/heads/rec_att_head.py

@@ -38,7 +38,7 @@ class AttentionHead(nn.Layer):
         return input_ont_hot
 
     def forward(self, inputs, targets=None, batch_max_length=25):
-        batch_size = inputs.shape[0]
+        batch_size = paddle.shape(inputs)[0]
         num_steps = batch_max_length
 
         hidden = paddle.zeros((batch_size, self.hidden_size))