Supporting subclassed pg.Functor.

Usage:

```python
import pyglove as pg

class Foo(pg.Functor):
  x: int
  y: int
  def _call(self) -> int:
    return self.x + self.y

Foo(1, 2)()    # Early bound.
Foo()(1, 2)    # Late bound.
Foo(1)(y=2)    # Partial bound.
Foo(1, 2)(y=3, override_args=True)   # Partial bound with override.
...
```

This CL also renames `pg.Functor.signature` to `pg.Functor.__signature__`.

PiperOrigin-RevId: 563788916

pyglove/core/patching/rule_based.py

@@ -137,7 +137,8 @@ def validate(self, x: symbolic.Symbolic) -> None:
   def __call__(
       self,
       x: symbolic.Symbolic
-      ) -> Union[Dict[str, Any], Tuple[Dict[str, Any], Callable[[], None]]]:
+      ) -> Union[Dict[str, Any],
+                 Tuple[Dict[str, Any], Callable[[Any], None]]]:
     """Override __call__ to get rebind dict."""
     return super().__call__(x, override_args=True)
 
@@ -218,7 +219,7 @@ def _decorator(fn):
     cls = functor_decorator(fn)
     _PATCHER_REGISTRY.register(name or fn.__name__,
                                typing.cast(Type[Patcher], cls))
-    arg_specs = cls.signature.args
+    arg_specs = cls.__signature__.args
     if len(arg_specs) < 1:
       raise TypeError(
           'Patcher function should have at least 1 argument '
@@ -337,7 +338,7 @@ def from_uri(uri: str) -> Patcher:
   """Create a Patcher object from a URI-like string."""
   name, args, kwargs = parse_uri(uri)
   patcher_cls = typing.cast(Type[Any], _PATCHER_REGISTRY.get(name))
-  args, kwargs = parse_args(patcher_cls.signature, args, kwargs)
+  args, kwargs = parse_args(patcher_cls.__signature__, args, kwargs)
   return patcher_cls(object_utils.MISSING_VALUE, *args, **kwargs)