register返回多个值和方法，这些值和方法传递给标准html输入
Spread是js语法：https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax
下面两个例子是相同的：

const { onChange, onBlur, name, ref } = register('firstName'); 
// include type check against field path with the name you have supplied.
        
<input 
  onChange={onChange} // assign onChange event 
  onBlur={onBlur} // assign onBlur event
  name={name} // assign name prop
  ref={ref} // assign ref prop
/>

// same as above
<input {...register('firstName')} />