add download script in image classification example

example/image-classification/pretraind_models/download_model.sh

+#!/bin/bash
+set -o nounset
+set -o errexit
+
+script_path=$(cd `dirname $0`; pwd)
+
+if [ $# -ne 1 ]
+then
+    echo "usage: sh $0 {PRETRAINED_MODEL_NAME}"
+    exit 1
+fi
+
+if [ $1 != "ResNet50" -a $1 != "ResNet101" -a $1 != "ResNet152" ]
+then
+    echo "only suppory pretrained model in {ResNet50, ResNet101, ResNet152}"
+    exit 1
+fi
+
+model_name=${1}_pretrained
+model=${model_name}.zip
+cd ${script_path}
+
+if [ -d ${model_name} ]
+then
+    echo "model file ${model_name} is already existed"
+    exit 0
+fi
+
+if [ ! -f ${model} ]
+then
+    wget http://paddle-imagenet-models-name.bj.bcebos.com/${model}
+fi
+unzip ${model}
+# rm ${model}
+rm -rf __MACOSX