LSTM

[akbir]

Adding an LSTMAgent.

This adds an LSTM option to the PPO Agents.

• Achieves parity on IPD

1. [x] Does your submission pass tests?
2. [x] Have you lint your code locally prior to submission?

Changes to Core Features:

• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [x] Have you written new tests for your core changes, as applicable?
• [x] Have you successfully ran tests with your changes locally?

[akbir]

@newtonkwan - can you pick this PR up and get into main before the release?