DeepChem Updates of April (Week 2), 2024

Here is a quick summary of merged pull requests for DeepChem during the second week of April:
1. Remove tutorial number from NormalizingFlow tutorial: This PR fixes formatting error.
2. Adding A2C class: This PR adds A2C class along with tests.
3. Adding loss classes for A2C: This PR adds A2C Discrete and A2C Continuous loss classes for A2C.