[Bug] [lang] Fix error when subscripting a dict

[archibate]

Related issue = #

A collection of several random yet trivial fixes.

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---

import taichi as ti

x = dict(a=1, b=2)

@ti.kernel
def func():
    ti.static_print(x['a'])

func()

---

[Taichi] mode=release
[Taichi] version 0.6.40, llvm 10.0.0, commit 78231814, win, python 3.8.5
[Taichi] materializing...
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-1-95603a473205> in <module>
      7     ti.static_print(x['a'])
      8 
----> 9 func()

~\AppData\Local\Programs\Python\Python38\lib\site-packages\taichi\lang\kernel.py in wrapped(*args, **kwargs)
    572         def wrapped(*args, **kwargs):
    573             _taichi_skip_traceback = 1
--> 574             return primal(*args, **kwargs)
    575 
    576         wrapped.grad = adjoint

~\AppData\Local\Programs\Python\Python38\lib\site-packages\taichi\lang\shell.py in new_call(*args, **kwargs)
     34     def new_call(*args, **kwargs):
     35         _taichi_skip_traceback = 1
---> 36         ret = old_call(*args, **kwargs)
     37         # print's in kernel won't take effect until ti.sync(), discussion:
     38         # https://github.com/taichi-dev/taichi/pull/1303#discussion_r444897102

~\AppData\Local\Programs\Python\Python38\lib\site-packages\taichi\lang\kernel.py in __call__(self, *args, **kwargs)
    499         instance_id, arg_features = self.mapper.lookup(args)
    500         key = (self.func, instance_id)
--> 501         self.materialize(key=key, args=args, arg_features=arg_features)
    502         return self.compiled_functions[key](*args)
    503 

~\AppData\Local\Programs\Python\Python38\lib\site-packages\taichi\lang\kernel.py in materialize(self, key, args, arg_features)
    368             self.runtime.inside_kernel = False
    369 
--> 370         taichi_kernel = taichi_kernel.define(taichi_ast_generator)
    371 
    372         assert key not in self.compiled_functions

~\AppData\Local\Programs\Python\Python38\lib\site-packages\taichi\lang\kernel.py in taichi_ast_generator()
    365                 )
    366             self.runtime.inside_kernel = True
--> 367             compiled()
    368             self.runtime.inside_kernel = False
    369 

<ipython-input-1-95603a473205> in func()
      5 @ti.kernel
      6 def func():
----> 7     ti.static_print(x['a'])
      8 
      9 func()

~\AppData\Local\Programs\Python\Python38\lib\site-packages\taichi\lang\util.py in wrapped(*args, **kwargs)
    197         assert in_taichi_scope(), \
    198                 f'{func.__name__} cannot be called in Python-scope'
--> 199         return func(*args, **kwargs)
    200 
    201     return wrapped

~\AppData\Local\Programs\Python\Python38\lib\site-packages\taichi\lang\impl.py in subscript(value, *indices)
    128         return Expr(taichi_lang_core.subscript(value.ptr, indices_expr_group))
    129     else:
--> 130         return value[indices]
    131 
    132 

KeyError: ('a',)

[archibate]

OFT further extend static-for possibilities.

[codecov]

Codecov Report

Merging #1950 into master will increase coverage by 0.84%.
The diff coverage is 45.45%.

@@            Coverage Diff             @@
##           master    #1950      +/-   ##
==========================================
+ Coverage   42.80%   43.64%   +0.84%     
==========================================
  Files          45       45              
  Lines        6413     6225     -188     
  Branches     1101     1106       +5     
==========================================
- Hits         2745     2717      -28     
+ Misses       3498     3334     -164     
- Partials      170      174       +4     

Impacted Files	Coverage Δ
python/taichi/lang/ndrange.py	73.52% <0.00%> (ø)
python/taichi/misc/gui.py	8.89% <16.66%> (ø)
python/taichi/lang/impl.py	66.75% <100.00%> (ø)
...ld/taichi-dev/taichi/python/taichi/tools/np2ply.py
.../taichi-dev/taichi/python/taichi/lang/exception.py
...uild/taichi-dev/taichi/python/taichi/misc/error.py
...avis/build/taichi-dev/taichi/python/taichi/main.py
...ild/taichi-dev/taichi/python/taichi/lang/kernel.py
...build/taichi-dev/taichi/python/taichi/lang/expr.py
...d/taichi-dev/taichi/python/taichi/core/__init__.py
... and 83 more

---

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[k-ye]

LGTM!

[yuanming-hu]

Cool! LGTM. Thank you!