[Bug Report] Slider input event should be triggered by moving the slider button instead of value change

### Element UI version
2.14.1

### OS/Browsers version
Windows 10 / Edge 87

### Vue version
2.6.12

### Reproduction Link
https://jsfiddle.net/rn41hs3g/

### Steps to reproduce
I know it was designed to work as it is, but I don't think the design is right in some cases like mine: I was trying to create input field that syncs with the slider in both ways without using slider's "show-input" props since its input style is quite limited. My implementation is having @input event fired to update value in Input field and have a computed "sliderValue" listening to the input-value change and return corresponding slider-value. It works when dragging the slider to update Input field value, but it would trigger the input-value to update twice and cause issues when entering value from Input field.

### What is Expected?
@input event should only fire when slider is dragged.

### What is actually happening?
@input event fires when slider's value has been changed.