add support for Chain-of-Thought

[Saibo-creator]

It would be great if we could support COT like below

grammar_str = """
root ::= cot | phone_number

cot ::= [ a-zA-Z0-9]* phone_number

phone_number ::= "+" [0-9]+
"""

or for arithemetic

root ::= cot | result

cot ::= [^[">>>"] ]* result # We exclude ">>>" from chain of thought to make it a special marker

result ::= ">>>" [0-9]+ 

This way, we could have a prompt like

Follow this example 3+3*4=3+12=15 >>>15; 97*(12-4)+9=

and expect a generation like 97*8+9=776+9=785 >>> 785

[Saibo-creator]

A draft version https://colab.research.google.com/drive/1RVMRdynb6FkScqCdfVHLJ6UeZfP2-bYT#scrollTo=hVal9tqz5hna