input: Write a python function to normalize tokens to lowercase.
  assert normalize_tokens(['Hello', 'WORLD', 'PyThOn']) == ['hello', 'world', 'python']
  assert normalize_tokens(['TEST', 'String', 'CaSe']) == ['test', 'string', 'case']
  assert normalize_tokens(['ALL', 'lower', 'CASE']) == ['all', 'lower', 'case']
output: def normalize_tokens(tokens):
      return [token.lower() for token in tokens]