[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/misc.py

@@ -274,3 +274,14 @@ def use_openmind() -> bool:
 
 def use_ray() -> bool:
     return is_env_enabled("USE_RAY")
+
+
+def find_available_port() -> int:
+    """Find an available port on the local machine."""
+    import socket
+
+    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+    sock.bind(("", 0))
+    port = sock.getsockname()[1]
+    sock.close()
+    return port