Revert removing the unused imports (#385)

python/sglang/api.py

@@ -3,7 +3,11 @@
 import re
 from typing import Callable, List, Optional, Union
 
+from sglang.backend.anthropic import Anthropic
 from sglang.backend.base_backend import BaseBackend
+from sglang.backend.openai import OpenAI
+from sglang.backend.runtime_endpoint import RuntimeEndpoint
+from sglang.backend.vertexai import VertexAI
 from sglang.global_config import global_config
 from sglang.lang.ir import (
     SglExpr,