[Bug Report] cascader组件修改v-model值不触发lazyLoad

### Element UI version
2.15.14

### OS/Browsers version
chrome

### Vue version
2.7.16

### code
```

let id = 0;
export default {
data() {
return {
input: '',
value: [],
props: {
lazy: true,
lazyLoad(node, resolve) {
const {
level
} = node;
setTimeout(() => {
const nodes = Array.from({
length: level + 1
})
.map(item => ({
value: ++id,
label: `选项${id}`,
leaf: level >= 2
}));
// 通过调用resolve将子节点数据返回，通知组件数据加载完成
resolve(nodes);
}, 1000);
}
},
}
},
methods: {
change(value) {
console.log(value);
let _this = this;
setTimeout(function(){

},500)
_this.value = [ 1, 2, 4 ]
}
},
}

```

### 问题现象