Release ProofAutograder and AnswerAutograder Prompt template

Hi @lmthang @dawsenhwang,

Thanks for the amazing contribution! This is Ziji from NUS, and we are intertested in using IMOBench in our ongoing ML System research project.

We notice that the prompt templates for `ProofAutograder` and `AnswerAutograder` were included in the original paper but not here. To facilitate reproduction, we think it will be helpful to include them. I will submit a seprate PR to incorperate them and hope that you may approve it.

Thanks!