We train $\mathcal {F}$ and $\mathcal {G}$ jointly and introduce two regularizers. Formally, we hope that $\mathcal {G}(\mathcal {F}(X))$ is similar to $X$ and $\mathcal {F}(\mathcal {G}(Y))$ is similar to $Y$. We implement this constraint as a cycle consistency loss. As a result, the proposed model has two learning objectives: i) an adversarial loss ($\ell _{adv}$) for each model as in the baseline. ii) a cycle consistency loss ($\ell _{cycle}$) on each side to avoid $\mathcal {F}$ and $\mathcal {G}$ from contradicting each other. 
 Question: What regularizers were used to encourage consistency in back translation cycles?
an adversarial loss ($\ell _{adv}$) for each model as in the baseline a cycle consistency loss ($\ell _{cycle}$) on each side