[Refactor] Improve scalar handling in CopyNode and update loop partition dtype logi (#1111)

* [Refactor] Improve scalar handling in CopyNode and update loop partition dtype logic

* Refactored CopyNode::MakeSIMTLoop to handle scalar cases more efficiently by moving the scalar check to the end of the function.
* Updated loop_partition.cc to set a default DataType for thread and vector extents, ensuring compatibility when loop_vars_ is empty.

* lint fix

* remove debug print

src/op/copy.cc

@@ -299,10 +299,6 @@ PrimExpr CopyNode::MakePredicate(arith::Analyzer *analyzer,
 For CopyNode::MakeSIMTLoop(arith::Analyzer *analyzer) const {
   Array<IterVar> loop_vars = MakeIterVars();
   bool is_scalar = loop_vars.empty();
-  if (is_scalar) {
-    return For(Var("i"), 0, 1, ForKind::kSerial,
-               BufferStore(dst, BufferLoad(src, {0}), {0}));
-  }
 
   for (const auto &iv : loop_vars)
     analyzer->Bind(iv->var, iv->dom);
@@ -332,6 +328,9 @@ For CopyNode::MakeSIMTLoop(arith::Analyzer *analyzer) const {
   Stmt body = BufferStore(dst, value, dst_indices);
   if (dst_predicate.defined())
     body = IfThenElse(dst_predicate, body);
+  if (is_scalar) {
+    return For(Var("i"), 0, 1, ForKind::kSerial, body);
+  }
   for (int i = loop_vars.size() - 1; i >= 0; i--) {
     Map<String, ObjectRef> annotations = {};
     if (coalesced_width.defined()) {

src/transform/loop_partition.cc

@@ -189,8 +189,10 @@ public:
 
   Fragment Partition(const For &op, int num_thread, int vectorize_size) {
     this->VisitStmt(op);
-    ICHECK(!loop_vars_.empty());
-    DataType dtype = loop_vars_[0]->var.dtype();
+    DataType dtype = DataType::Int(32);
+    if (!loop_vars_.empty()) {
+      dtype = loop_vars_.back()->var.dtype();
+    }
     PrimExpr flattened = make_const(dtype, 0);
     PrimExpr vector_extent = make_const(dtype, vectorize_size);
     PrimExpr thread_extent_const = make_const(dtype, num_thread);