Release the pretrained model

fluid/DeepASR/examples/aishell/download_pretrained_model.sh

+url=http://deep-asr-data.gz.bcebos.com/aishell_pretrained_model.tar.gz
+md5=7b51bde64e884f43901b7a3461ccbfa3
+
+wget -c $url
+
+echo "Checking md5 sum ..."
+md5sum_tmp=`md5sum aishell_pretrained_model.tar.gz | cut -d ' ' -f1`
+
+if [ $md5sum_tmp !=  $md5 ]; then
+    echo "Md5sum check failed, please remove and redownload "
+          "aishell_pretrained_model.tar.gz."
+    exit 1
+fi
+
+tar xvf aishell_pretrained_model.tar.gz