[bug report] when the value of slider is set, the 'input' event will be triggered

### Element UI version
2.15.6

### OS/Browsers version
Microsoft Edge 版本 94.0.992.31 (官方内部版本) (64 位)

### Vue version
2.6.14

### Reproduction Link
https://codepen.io/windlike/pen/LYLapJq

### Steps to reproduce
1. 点击重置按钮

### What is Expected?
设置值时不会触发`input`事件，手动拖动Slider时才触发

### What is actually happening?
设置值时会触发`input`事件