support speculative execution for openai API (#48)

Co-authored-by: Ying Sheng <sqy1415@gmail.com>

examples/usage/openai_speculative.py

@@ -0,0 +1,19 @@
+from sglang import function, gen, set_default_backend, OpenAI
+
+
+@function(api_num_spec_tokens=512)
+def gen_character_spec(s):
+    s += "Construct a character within the following format:\n"
+    s += "Name: Steve Jobs.\nBirthday: February 24, 1955.\nJob: Apple CEO.\n"
+    s += "\nPlease generate new Name, Birthday and Job.\n"
+    s += "Name:" + gen("name", stop="\n") + "\nBirthday:" + gen("birthday", stop="\n")
+    s += "\nJob:" + gen("job", stop="\n") + "\n"
+
+
+set_default_backend(OpenAI("gpt-3.5-turbo-instruct"))
+
+state = gen_character_spec.run()
+
+print("name:", state["name"])
+print("birthday:", state["birthday"])
+print("job:", state["job"])