[Script] Be more careful when generating ast.ExtSlice for Subscript

[kparzysz-quic]

The ast.ExtSlice expects a non-empty list, otherwise evaluation fails with error: empty dims on ExtSlice. Also, each element in "dims" list of ExtSlice must be either Slice or Index.

In python3.8 an expression A[()] is parsed (by ast) as Subscript with slice being Index(value=Tuple(elts=[])). When we translate a subscript from doc.AST to ast, we unconditionally convert every tuple to ast.ExtSlice, which in this case is incorrect.

The fix is to map empty tuple back to the Index(Tuple[])) instead of ExtSlice. In other cases, ensure that members of ExtSlice are of correct types.

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[tvm-bot]

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[tvm-bot]

Thanks for contributing to TVM! Please refer to the contributing guidelines https://tvm.apache.org/docs/contribute/ for useful information and tips. Please request code reviews from Reviewers by @-ing them in a comment.

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[kparzysz-quic]

cc: @Hzfengsy @junrushao @yzh119

[Lunderberg]

Thank you for the fix, and LGTM! It looks like this is specifically for python 3.8 and earlier, as the ast.Index and ast.ExtSlice classes were deprecated in python 3.9.

I'm rather surprised on how this test was able to pass, as the CI runs on python3.8.16, and this test case also uses length-zero tuples as an index. Does the bug only trigger when the object is passed to an external macro?

[kparzysz-quic]

This happens when we evaluate and expression containing () as an index, specifically when it get translated back into ast and then passed to eval. I encountered this while using macros, and I also thought it should happen without macros too, but I wasn't able to reproduce it with a simple testcase. I can dig into this some more to see why it worked there, but failed in a macro.

[junrushao]

Interesting findings! When initiating TVMScript, we tested bunch of cases in certain python versions, but not all - which might be the source of such issues