DeepChem Updates of April (Week 4), 2024

Here is a quick summary of merged pull requests for DeepChem during the fourth week of April:

1. DFT Part - 6: This PR updates DFT.
2. Adding PPO class: This PR ports PPO from Tensorflow to PyTorch.
3. fix#3746 fit_generator() not accepting variables as generators: This PR fixes an issue(3746) that arise when model.parameters() is consumed before being passed to fit_generator(), resulting in a ValueError: optimizer got an empty parameter list.
4. Flows - Affine and Masked Affine: This PR adds an abstract class for flow-based models, adds Affine Flow to the flows, and a new layer, Masked Affine Flow.
5. Adding polymer tutorial: This PR adds an ipython notebook for the polymer representation tutorial.