[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>

src/llamafactory/extras/constants.py

@@ -106,6 +106,7 @@ class AttentionFunction(str, Enum):
 class EngineName(str, Enum):
     HF = "huggingface"
     VLLM = "vllm"
+    SGLANG = "sglang"
 
 
 class DownloadSource(str, Enum):