input: Write a function to detect and autofix errors in a given Python code using the ast module.
  assert detect_errors("def test():\n return") == ["Return statement should be followed by an expression"]
  assert detect_errors("def test():\n a = 1\n return a") == ["Name node 'a' should be declared"]
  assert autofix("def test():\n return") != ""
output: import ast
  
  def detect_errors(code):
      errors = []
      
      class CodeVisitor(ast.NodeVisitor):
          def visit_Return(self, node):
              if node.value is None:
                  errors.append("Return statement should be followed by an expression")
              self.generic_visit(node)
      
      try:
          tree = ast.parse(code)
          CodeVisitor().visit(tree)
      except SyntaxError as e:
          errors.append(str(e))
      
      return errors
  
  def autofix(code):
      errors = detect_errors(code)
      if not errors:
          return code
      
      fixed_code = code
      if "Return statement should be followed by an expression" in errors:
          fixed_code = code.replace("return", "return None")
      
      return fixed_code