Duplicate `execute_code` function in demo/backend.py

[lordk911]

here are two top-level definitions of execute_code(code_str) in demo/backend.py:

• at demo/backend.py:41
  https://github.com/ruc-datalab/DeepAnalyze/blob/9f594964972fec82a2fb243b31541700555eabe8/demo/backend.py#L41C1-L41C28

• at demo/backend.py:251
  https://github.com/ruc-datalab/DeepAnalyze/blob/9f594964972fec82a2fb243b31541700555eabe8/demo/backend.py#L251C1-L251C28