[TE] Promote substituted variable to iter_var's dtype

[tkonolige]

This fixes a bug where an iteration variable and its associated loop variable have a mismatched dtype.

@mbrookhart@junrushao1994

[tkonolige]

This problem is a lot more complicated than I initially thought. It is actually two problems: 1. IterVar has an invariant that the dtype of its var and the dtype of its domain's extent (dom->extent) match. I fixed all violations of this invariant and added a check for it in the constructor of IterVar. 2. The IterVar of thread/block and the IterVar it binds to need to have the same dtype when computing loop nesting. However, there is no way to know what datatype is needed when constructing the thread/block IterVar, so it defaults to int32. I've added an automatic promotion from thread/block IterVar dtype to the bound IterVar's dtype when computing loop nests, but this is a hack and will cause integer overflow if the bound IterVar exceeds int32::max (should not happen unless we get a GPU/CPU with more than int32::max threads).

[masahi]

Interesting, does this change remove the need for fixes in the following PRs?

#10519
#9582
#10172

I've been trying to compile the QAT BERT from mlperf via various ways but I consistently get the int32 vs int64 dtype mismatch like:

 2: tvm::arith::SolveInequalitiesToRange(tvm::arith::IntConstraints const&)
1: tvm::arith::AsConditions(tvm::runtime::Array<tvm::tir::Var, void> const&, tvm::runtime::Map<tvm::tir::Var, tvm::arith::IntGroupBounds, void, void> const&, tvm::runtime::Array<tvm::PrimExpr, void> const&)
0: tvm::tir::LE::LE(tvm::PrimExpr, tvm::PrimExpr, tvm::Span)
File "/home/masa/projects/dev/tvm/src/tir/ir/expr.cc", line 447
TypeError: Check failed: (a.dtype() == b.dtype()) is false: mismatched types. int32 vs. int64

 1: tvm::tir::ExprMutator::VisitExpr_(tvm::tir::FloorDivNode const*)
0: tvm::tir::FloorDiv::FloorDiv(tvm::PrimExpr, tvm::PrimExpr, tvm::Span)
File "/home/masa/projects/dev/tvm/src/tir/ir/expr.cc", line 322
TypeError: Check failed: (a.dtype() == b.dtype()) is false: mismatched types. int64 vs. int32

 1: tvm::tir::ArgBinder::BindBuffer(tvm::tir::Buffer const&, tvm::tir::Buffer const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, bool)
0: tvm::tir::ArgBinder::Bind_(tvm::PrimExpr const&, tvm::PrimExpr const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, bool)
File "/home/masa/projects/dev/tvm/src/tir/transforms/arg_binder.cc", line 52
TVMError:
---------------------------------------------------------------
An error occurred during the execution of TVM.
For more information, please see: https://tvm.apache.org/docs/errors.html
---------------------------------------------------------------
Check failed: arg.dtype() == value.dtype() (int32 vs. int64) :

[tkonolige]

I think it may fix the middle and top issues, but I'm not positive.

Also, we should think about checking that every For also has matching dtypes between its var and extent.

[masahi]

just sent a PR fixing a related problem (one of my errors above)

#10584

[mbrookhart]

@junrushao1994@masahi Any concerns with this?