input: Write a function to parse an XML document and return a list of tag objects. Each tag object should have an id attribute and a list of values, where each value corresponds to an attribute of a subchild tag.
  
  
  assert parse_xml("""<xml>
    <root>
      <child1 id="1">
        <subchild1 value="a">
        </subchild1>
        <subchild2 value="b">
        </subchild2>
      </child1>
      <child2 id="2">
        <subchild3 value="c">
        </subchild3>
        <subchild4 value="d">
        </subchild4>
      </child2>
    </root>
  </xml>""") == [Tag(tag_id="1", values=["a", "b"]), Tag(tag_id="2", values=["c", "d"])]
  
output: def parse_xml(xml_str):
      from xml.etree import ElementTree as ET
      root = ET.fromstring(xml_str)
      tags = []
      for child in root.findall('.//root/*'):
          tag_id = child.get('id')
          values = [subchild.get('value') for subchild in child.findall('./*')]
          tags.append(Tag(tag_id=tag_id, values=values))
      return tags