[TGA] Factual Nucleus Sampling

[klshuster]

Patch description Factual nucleus is a decoding method that decays the nucleus sampling p value over time according to a constant factor lambda_decay. The p value resets when encountering a full-stop (when p_reset is True), and there is a lower bound of omega_bound to what p can decay to.

Testing steps Added Factual Nucleus to the existing tree search tests.