[Bug Report] el-date-picker component, problems occurs when using “format” attribute with escape characters on type “datetime”

### Element UI version
2.11.0

### OS/Browsers version
os: win10 browser: chrome 88.0.4324.150

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/jodieShi/pen/RwoRMmd

### Steps to reproduce
1. binding a el-date-picker with type “datetime” and format "yyyy 年 MM 月 dd 日 HH 时 mm 分 ss 秒"
2. click to select
3. select any datetime value, such as: "2021-02-09 00:00:00"

### What is Expected?
correct selected value on date and time input correspondingly with expected format.
In the case of "2021-02-09 00:00:00", it is expected to be "2021 年 02 月 09 日 " and "00 时 00 分 00 秒" correspondingly.

### What is actually happening?
The values of date and time input are "2021 年 02 月 09 日 时 分 秒" and "年 月 日 00 时 00 分 00 秒".