input: Write a python function to fetch all HTML elements from a given URL and return them as a list of strings.
  assert fetch_html_elements('https://www.example.com/') != []
  assert isinstance(fetch_html_elements('https://www.example.com/'), list)
  assert all(isinstance(element, str) for element in fetch_html_elements('https://www.example.com/'))
output: def fetch_html_elements(url):
      import requests
      from bs4 import BeautifulSoup
  
      response = requests.get(url)
      soup = BeautifulSoup(response.content, 'html.parser')
      elements = [str(tag) for tag in soup.find_all(True)]
      return elements