Typo: Fix generate signature type hint for attention_backend

`attention_backend` is a SDPBackend, not a string

Author: Jack-Khuu

torchchat/generate.py

@@ -685,7 +685,7 @@ def generate(
         sequential_prefill=True,
         callback=lambda x: x,
         max_seq_length: int,
-        attention_backend: str = "math",
+        attention_backend: SDPBackend = torch.nn.attention.SDPBackend.MATH,
         seed: Optional[int] = None,
         **sampling_kwargs,
     ) -> torch.Tensor: