[mlir][sparse] replace magic constant with symbol

mlir/lib/Dialect/SparseTensor/Transforms/SparseTensorCodegen.cpp

 #include "mlir/Dialect/Tensor/IR/Tensor.h"
 #include "mlir/Transforms/DialectConversion.h"
 
 using namespace mlir;
 using namespace mlir::sparse_tensor;
 
 namespace {
 
-// TODO: start using these when insertions are implemented
-// static constexpr uint64_t DimSizesIdx = 0;
-// static constexpr uint64_t DimCursorIdx = 1;
+static constexpr uint64_t DimSizesIdx = 0;
 static constexpr uint64_t MemSizesIdx = 2;
 static constexpr uint64_t FieldsIdx = 3;
 
 //===----------------------------------------------------------------------===//
 // Helper methods.
 //===----------------------------------------------------------------------===//
 
 /// Returns the "tuple" value of the adapted tensor.
if (!enc)
     return llvm::None;
 
   // Access into static dimension can query original type directly.
   // Note that this is typically already done by DimOp's folding.
   auto shape = tensorTp.getShape();
   if (!ShapedType::isDynamic(shape[dim]))
     return constantIndex(rewriter, loc, shape[dim]);
 
-  // Any other query can consult the dimSizes array at field 0 using,
+  // Any other query can consult the dimSizes array at field DimSizesIdx,
   // accounting for the reordering applied to the sparse storage.
   auto tuple = getTuple(adaptedValue);
   Value idx = constantIndex(rewriter, loc, toStoredDim(tensorTp, dim));
-  return rewriter.create<memref::LoadOp>(loc, tuple.getInputs().front(), idx)
+  return rewriter
+      .create<memref::LoadOp>(loc, tuple.getInputs()[DimSizesIdx], idx)
       .getResult();
 }
 
 /// Translates field index to memSizes index.
 static unsigned getMemSizesIndex(unsigned field) {
   assert(FieldsIdx <= field);
   return field - FieldsIdx;
 }