pg.Object: Fix an issue that overridden property is not correctly i…

…nherited by subclasses.

Example:

```
import pyglove as pg

class A(pg.Object):
  x: int = 1

class B(A):
  @Property
  def x(self) -> int:
    return 2

class C(B):
  pass
```

Previously `C.x` will be 1 (the value of symbolic field x), after this change C.x will be 2 (the value of overriden property x inherited from B).

PiperOrigin-RevId: 615494844

pyglove/core/symbolic/object.py

@@ -381,7 +381,7 @@ def _generate_sym_attributes(cls):
     for key, field in cls.__schema__.fields.items():
       if isinstance(key, pg_typing.ConstStrKey):
         attr_name = str(key)
-        attr_value = cls.__dict__.get(attr_name, pg_typing.MISSING_VALUE)
+        attr_value = getattr(cls, attr_name, pg_typing.MISSING_VALUE)
         if attr_value == pg_typing.MISSING_VALUE or (
             not inspect.isfunction(attr_value)
             and not isinstance(attr_value, property)

pyglove/core/symbolic/object_test.py

@@ -398,6 +398,14 @@ def z(self):
     self.assertEqual(a.z(), 5)
     self.assertEqual(a.sym_init_args.z, 3)
 
+    # Make sure overridden property is inherited by subclass correctly.
+    class B(A):
+      pass
+
+    b = B(1, 2, 3)
+    self.assertEqual(b.x, 2)
+    self.assertEqual(b.sym_init_args.x, 1)
+
   def test_runtime_type_check(self):
 
     @pg_members([