[Lang] [bug] Let nested data classes have methods (#7909)

Issue: fixes #7908 
Added the methods in the string representation of the class as well.

python/taichi/lang/struct.py

@@ -179,6 +179,7 @@ def __str__(self):
         """Python scope struct array print support."""
         if impl.inside_kernel():
             item_str = ", ".join([str(k) + "=" + str(v) for k, v in self.items])
+            item_str += f", struct_methods={self.methods}"
             return f"<ti.Struct {item_str}>"
         return str(self.to_dict())
 
@@ -196,7 +197,11 @@ def to_dict(self, include_methods=False, include_ndim=False):
             Dict: The result dictionary.
         """
         res_dict = {
-            k: v.to_dict() if isinstance(v, Struct) else v.to_list() if isinstance(v, Matrix) else v
+            k: v.to_dict(include_methods=include_methods, include_ndim=include_ndim)
+            if isinstance(v, Struct)
+            else v.to_list()
+            if isinstance(v, Matrix)
+            else v
             for k, v in self.entries.items()
         }
         if include_methods:

tests/python/test_struct.py

@@ -128,3 +128,29 @@ def foo(x: Foo) -> Foo:
     c = b.add_python(b)
     assert c.x == 4
     assert c.y == 8
+
+
+@test_utils.test()
+def test_nested_data_class_func():
+    @ti.dataclass
+    class Foo:
+        a: int
+
+        @ti.func
+        def foo(self):
+            return self.a
+
+    @ti.dataclass
+    class Nested:
+        f: Foo
+
+        @ti.func
+        def testme(self) -> int:
+            return self.f.foo()
+
+    @ti.kernel
+    def k() -> int:
+        x = Nested(Foo(42))
+        return x.testme()
+
+    assert k() == 42