[mlir][sparse] add source materizalization callback for sparse tensor codegen type converter.

Required by scf.for to achieve 1:N type conversion (See D136314).

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D136367

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

@@ -745,6 +745,17 @@ public:
 mlir::SparseTensorTypeToBufferConverter::SparseTensorTypeToBufferConverter() {
   addConversion([](Type type) { return type; });
   addConversion(convertSparseTensorType);
+
+  // Required by scf.for 1:N type conversion.
+  addSourceMaterialization([](OpBuilder &builder, RankedTensorType tp,
+                              ValueRange inputs,
+                              Location loc) -> Optional<Value> {
+    if (!getSparseTensorEncoding(tp))
+      // Not a sparse tensor.
+      return llvm::None;
+    // Sparse compiler knows how to cancel out these casts.
+    return genTuple(builder, loc, tp, inputs);
+  });
 }
 
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