TYP: dual typing fixes (#543)

Co-authored-by: JHM Darbyshire (win11) <[email protected]>

python/rateslib/dual/__init__.py

@@ -131,13 +131,16 @@ def gradient(
             return dual.dual
         elif vars is not None and not keep_manifold:
             return dual.grad1(vars)
-
-        _ = dual.grad1_manifold(vars)
+        elif isinstance(dual, Dual): # and keep_manifold:
+            raise TypeError("Dual type cannot perform `keep_manifold`.")
+        _ = dual.grad1_manifold(dual.vars if vars is None else vars)
         return np.asarray(_)
 
     elif order == 2:
         if isinstance(dual, Variable):
             dual = Dual2(dual.real, vars=dual.vars, dual=dual.dual, dual2=[])
+        elif isinstance(dual, Dual):
+            raise TypeError("Dual type cannot derive second order automatic derivatives.")
 
         if vars is None:
             return 2.0 * dual.dual2

python/rateslib/rs.pyi

@@ -189,7 +189,7 @@ class Dual2:
         dual2: list[float] | Arr1dF64,
     ) -> Dual2: ...
     def grad1(self, vars: Sequence[str]) -> Arr1dF64: ...
-    def gra1_manifold(self, vars: Sequence[str]) -> list[Dual2]: ...
+    def grad1_manifold(self, vars: Sequence[str]) -> list[Dual2]: ...
     def grad2(self, vars: list[str]) -> Arr2dF64: ...
     def ptr_eq(self, other: Dual2) -> bool: ...
     def __repr__(self) -> str: ...