type object 'Expr' has no attribute 'layout_materialized'

[vkurenkov]

Describe the bug
The autodiff_minimization.py/autodiff_regression.py examples are not working in 0.6.13 version but working in 0.6.12

Log/Screenshots

[Taichi] mode=release
[Taichi] version 0.6.13, llvm 10.0.0, commit e8d87357, python 3.7.6
Traceback (most recent call last):
  File "autodiff_minimization.py", line 18, in <module>
    x[i] = random.random()
  File "/home/vkurenkov/.anaconda3/lib/python3.7/site-packages/taichi/lang/util.py", line 212, in wrapped
    return func(*args, **kwargs)
  File "/home/vkurenkov/.anaconda3/lib/python3.7/site-packages/taichi/lang/expr.py", line 39, in __setitem__
    impl.get_runtime().try_materialize()
  File "/home/vkurenkov/.anaconda3/lib/python3.7/site-packages/taichi/lang/impl.py", line 166, in try_materialize
    if not Expr.layout_materialized:
AttributeError: type object 'Expr' has no attribute 'layout_materialized'

To Reproduce

# Install target version
pip install taichi==0.6.13

# Pull the example
git clone https://github.com/taichi-dev/taichi.git
cd taichi/examples/

# Run the example
python autodiff_minimization.py

[yuanming-hu]

Thanks for reporting. This can be fixed by adding ti.init() at the beginning of the example. It's also worth checking if an ti.init is missing in any other examples.

If anyone's interested in fixing this, please go ahead and PR. Thanks in advance!