Fix Mistral model loading (#108)

Co-authored-by: johndun <dunavent.jm@gmail.com>

python/sglang/srt/models/mistral.py

+"""Inference-only Mistral model."""
+from sglang.srt.models.llama2 import LlamaForCausalLM
+
+
+class MistralForCausalLM(LlamaForCausalLM):
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+
+
+EntryClass = MistralForCausalLM