add head_dim

Fix bug of vllm version 0.8.5:
```
self.q_size = self.num_heads * self.head_dim
TypeError: unsupported operand type(s) for *: 'int' and 'NoneType'
```

config.json

@@ -14,7 +14,7 @@
       "MistralForCausalLM"
     ],
     "attention_dropout": 0.0,
-    "head_dim": null,
+    "head_dim": 128,
     "hidden_act": "silu",
     "hidden_size": 4096,
     "initializer_range": 0.02,