Ported back new grammar changes from C++ to Python implementation

llama_cpp/llama_cpp.py

@@ -3002,7 +3002,7 @@ def llama_grammar_init(
     n_rules: Union[ctypes.c_size_t, int],
     start_rule_index: Union[ctypes.c_size_t, int],
     /,
-) -> llama_grammar_p:
+) -> Optional[llama_grammar_p]:
     """Initialize a grammar from a set of rules."""
     ...