fix(101): align KL divergence calculation with GRPO paper and fix test

[cyk1337]

• Problem: The current implementation in question 101 computes a standard KL divergence, which does not match the mathematical formulation defined in the original GRPO paper. Also, the expected outputs did not match solution results on the website.
• Solution:
  1. Updated the grpo_objective function to implement the calculation as described in the GRPO paper (https://arxiv.org/pdf/2402.03300).
  2. Fixed the corresponding test cases to match the corrected implementation.
• Impact: This fix ensures the correctness of the GRPO algorithm's core component.

[cyk1337]

@moe18 @Open-Deep-ML