使Bootstrap输入在移动设备上垂直对齐，在桌面上水平对齐

<div class="container">
  <div class="row">
    <div class="col-md-6">
      <div class="form-group">
        <label for="input1">输入框1</label>
        <input type="text" class="form-control" id="input1">
      </div>
      <div class="form-group">
        <label for="input2">输入框2</label>
        <input type="text" class="form-control" id="input2">
      </div>
    </div>
    <div class="col-md-6">
      <div class="form-group">
        <label for="input3">输入框3</label>
        <input type="text" class="form-control" id="input3">
      </div>
      <div class="form-group">
        <label for="input4">输入框4</label>
        <input type="text" class="form-control" id="input4">
      </div>
    </div>
  </div>
</div>

<style>
  @media (max-width: 767px) {
    .vertical-align-input {
      display: flex;
      flex-direction: column;
      align-items: flex-start;
    }
  }
  
  @media (min-width: 768px) {
    .horizontal-align-input {
      display: flex;
      flex-direction: row;
      align-items: center;
    }
  }
</style>

<div class="container">
  <div class="vertical-align-input">
    <div class="form-group">
      <label for="input1">输入框1</label>
      <input type="text" class="form-control" id="input1">
    </div>
    <div class="form-group">
      <label for="input2">输入框2</label>
      <input type="text" class="form-control" id="input2">
    </div>
  </div>
  <div class="horizontal-align-input">
    <div class="form-group">
      <label for="input3">输入框3</label>
      <input type="text" class="form-control" id="input3">
    </div>
    <div class="form-group">
      <label for="input4">输入框4</label>
      <input type="text" class="form-control" id="input4">
    </div>
  </div>
</div>