[v1.7.0][Bug] Fix Diff Taichi error with "liquid.py"

[jim19930609]

Reproduce

cd difftaichi/examples
python3 diffmpm_benchmark.py

Log

[Taichi] version 1.7.0, llvm 15.0.4, commit fd46a563, linux, python 3.10.12
[Taichi] Starting on arch=cuda
Traceback (most recent call last):
  File "/home/taichigraphics/workspace/TaichiRepos/Release/difftaichi/examples/liquid.py", line 387, in <module>
    def copy_back_and_clear(img: ti.ext_arr()):
  File "/home/taichigraphics/workspace/TaichiRepos/Release/taichi/python/taichi/__init__.py", line 23, in __getattr__
    raise AttributeError(f"module '{__name__}' has no attribute '{attr}'")
AttributeError: module 'taichi' has no attribute 'ext_arr'

[jim19930609]

Fix in: taichi-dev/difftaichi#68