[lang] Raise an error when struct-for indices number mismatch

[archibate]

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Before this PR, the following code won't raise any error at all:

x = ti.var(ti.f32, (3, 4))

@ti.kernel
def func():
    for i in x:  # either use `for i, j in x` or `for I in ti.grouped(x)` instead!
        print(i)

func()

So I'm making struct-for mismatch raise IndexError now.

But I apply the same to ndrange-for, cause it seems ti.ndrange transformer needs refactoring anyway... IMO it should share the same error checker if we decide to do so.

I think just alerting mismatch struct-for and saying maybe you want to use ti.grouped() instead is good enough, since user can see range numbers if using ndrange-for.

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Btw, I think we may default to ti.grouped at this case?

[yuanming-hu]

Thanks! LGTM except for some wording issues.

Btw, I think we may default to ti.grouped at this case?

That sounds too smart to me. I'd rather keep everything explicit :-)

[codecov]

Codecov Report

Merging #1357 into master will decrease coverage by 1.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1357      +/-   ##
==========================================
- Coverage   66.51%   65.40%   -1.12%     
==========================================
  Files          37       37              
  Lines        5149     5009     -140     
  Branches      932      934       +2     
==========================================
- Hits         3425     3276     -149     
- Misses       1562     1571       +9     
  Partials      162      162              

Impacted Files	Coverage Δ
python/taichi/lang/transformer.py	93.54% <ø> (-0.10%)	⬇️
python/taichi/lang/impl.py	89.93% <100.00%> (-0.17%)	⬇️
python/taichi/misc/image.py	56.00% <0.00%> (-11.57%)	⬇️
python/taichi/misc/task.py	75.00% <0.00%> (-5.00%)	⬇️
python/taichi/lang/ast_checker.py	70.58% <0.00%> (-1.64%)	⬇️
python/taichi/lang/ops.py	92.30% <0.00%> (-1.45%)	⬇️
python/taichi/main.py	40.42% <0.00%> (-1.03%)	⬇️
python/taichi/lang/expr.py	89.10% <0.00%> (-0.72%)	⬇️
python/taichi/misc/gui.py	23.94% <0.00%> (-0.71%)	⬇️
python/taichi/lang/matrix.py	90.03% <0.00%> (-0.68%)	⬇️
... and 8 more

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