add a small feature in web_demo

[thkkk]

Added the function of pressing the enter key to generate an answer in web_demo

[lsvih]

This implementation will result in the predict function being called twice if the enter key and generate button are simultaneously pressed.

[duzx16]

I think this will confuse users if they want to give multi-line query.

[mengxinyuan638]

In my personal opinion, it's enough to have a button to generate an answer, but returning the answer by pressing Enter may cause unnecessary trouble