support speculative execution for openai API

[parasol-aser]

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[parasol-aser]

this PR supports online speculative execution. the basic idea is to track what's returned in previous gen calls and match with the prefix text

• if there is a match, reuse the completion and correct the name mapping

• in the test openai_example_speculative.py, now OPENAI API is called only once, instead of three times

Construct a character. Here is an example:
Name: Steve Jobs. Birthday: February 24, 1955. Job: Apple CEO.
Please generate new Name, Birthday and Job.
Name:  Maya Rodriguez
Birthday: October 12, 1989
Job: Graphic