[TVMScript] Produce empty DictAttrs when R.func_attrs is absent (#16844)

A follow-up to #16745.  For Relax
functions produced in TVMScript, when `R.func_attrs` was not present,
the default was set to `None` instead of an empty dictionary.

Author: Lunderberg

src/relax/ir/expr.cc

@@ -493,6 +493,10 @@ TVM_REGISTER_NODE_TYPE(FunctionNode);
 
 Function::Function(Array<Var> params, Expr body, Optional<StructInfo> ret_struct_info, bool is_pure,
                    DictAttrs attrs, Span span) {
+  if (!attrs.defined()) {
+    attrs = DictAttrs();
+  }
+
   // Set the function type.
   // For function, we take a conservative approach and require the function type
   // to be known at construction time.

src/script/ir_builder/relax/frame.cc

@@ -61,13 +61,12 @@ void FunctionFrameNode::ExitWithScope() {
       !attrs.count(tvm::attr::kGlobalSymbol)) {
     attrs.Set(tvm::attr::kGlobalSymbol, name.value());
   }
-  auto dict_attrs = attrs.empty() ? NullValue<DictAttrs>() : DictAttrs(attrs);
   this->block_builder->EndScope();
   tvm::relax::Function func(/*params=*/params,
                             /*body=*/body,
                             /*ret_struct_info=*/ret_struct_info,
                             /*is_pure=*/is_pure.value_or(Bool(true))->value,
-                            /*attrs=*/dict_attrs);
+                            /*attrs=*/DictAttrs(attrs));
   // Step 2: Update IRModule.
   if (builder->frames.empty()) {
     // Case 0. No outer frame, return function directly

src/tir/ir/function.cc

@@ -70,6 +70,10 @@ relax::StructInfo InferStructInfo(const PrimFunc& prim_func) {
 // Get the function type of a PrimFunc
 PrimFunc::PrimFunc(Array<tir::Var> params, Stmt body, Type ret_type,
                    Map<tir::Var, Buffer> buffer_map, DictAttrs attrs, Span span) {
+  if (!attrs.defined()) {
+    attrs = DictAttrs();
+  }
+
   // Assume void-return type for now
   // TODO(tvm-team) consider type deduction from body.
   if (!ret_type.defined()) {

tests/python/relax/test_tvmscript_parser.py

@@ -2271,5 +2271,27 @@ def main(A: R.Tensor, B: R.Tensor):
     tvm.ir.assert_structural_equal(DefinedAllAtOnce, MainDefinedLater)
 
 
+def test_function_attributes_are_defined():
+    """func.attrs defaults to an empty DictAttrs"""
+
+    @I.ir_module
+    class Module:
+        @R.function
+        def main(x: R.Tensor, shape: R.Shape(["m", "n"])):
+            output = Module.subroutine(x, shape)
+            return output
+
+        @R.function
+        def subroutine(x: R.Tensor, _: R.Shape(["m", "n"])) -> R.Tensor(["m", "n"]):
+            q = x
+            m, n = T.int64(), T.int64()
+            z = R.match_cast(q, R.Tensor((m, n)))
+            w = z
+            return w
+
+    for gvar, func in Module.functions.items():
+        assert func.attrs is not None
+
+
 if __name__ == "__main__":
     tvm.testing.main()