input: Write a python function to convert a CSV string to a DataFrame.
  assert csv_to_df("col1,col2\nval1,val2\nval3,val4").equals(pd.DataFrame({'col1': ['val1', 'val3'], 'col2': ['val2', 'val4']}))
  assert csv_to_df("A,B,C\n1,2,3\n4,5,6").equals(pd.DataFrame({'A': ['1', '4'], 'B': ['2', '5'], 'C': ['3', '6']}))
  assert csv_to_df("Name,Age,Location\nJohn,30,USA\nJane,25,Canada").equals(pd.DataFrame({'Name': ['John', 'Jane'], 'Age': ['30', '25'], 'Location': ['USA', 'Canada']}))
output: import pandas as pd
  from io import StringIO
  
  def csv_to_df(csv_string):
      return pd.read_csv(StringIO(csv_string))