[inference] support sglang backend (#7278)

* Mimic SGLang offline Engine

* Add more tests and args

* Pass all current tests

* Clean Code

* fix sample_params

* clean code

* Fix Stream Chat

* change sglang from engine mode to server mode

* fix

* Fix Review Issues

* Use SGLang Built-In Utilities

* Fix test SGLang

* Some Doc Issue

* fix sglang engine

* add readme

---------

Co-authored-by: Jin Pan <jpan236@wisc.edu>
Co-authored-by: hiyouga <hiyouga@buaa.edu.cn>

tests/e2e/test_sglang.py

@@ -0,0 +1,71 @@
+# Copyright 2025 the LlamaFactory team.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import sys
+
+import pytest
+
+from llamafactory.chat import ChatModel
+from llamafactory.extras.packages import is_sglang_available
+
+
+MODEL_NAME = "meta-llama/Llama-3.2-1B-Instruct"
+
+
+INFER_ARGS = {
+    "model_name_or_path": MODEL_NAME,
+    "finetuning_type": "lora",
+    "template": "llama3",
+    "infer_dtype": "float16",
+    "infer_backend": "sglang",
+    "do_sample": False,
+    "max_new_tokens": 1,
+}
+
+
+MESSAGES = [
+    {"role": "user", "content": "Hi"},
+]
+
+
+@pytest.mark.skipif(not is_sglang_available(), reason="SGLang is not installed")
+def test_chat():
+    r"""Test the SGLang engine's basic chat functionality."""
+    chat_model = ChatModel(INFER_ARGS)
+    response = chat_model.chat(MESSAGES)[0]
+    # TODO: Change to EXPECTED_RESPONSE
+    print(response.response_text)
+
+
+@pytest.mark.skipif(not is_sglang_available(), reason="SGLang is not installed")
+def test_stream_chat():
+    r"""Test the SGLang engine's streaming chat functionality."""
+    chat_model = ChatModel(INFER_ARGS)
+
+    response = ""
+    for token in chat_model.stream_chat(MESSAGES):
+        response += token
+
+    print("Complete response:", response)
+    assert response, "Should receive a non-empty response"
+
+
+# Run tests if executed directly
+if __name__ == "__main__":
+    if not is_sglang_available():
+        print("SGLang is not available. Please install it.")
+        sys.exit(1)
+
+    test_chat()
+    test_stream_chat()