Credit-assigned Policy Gradient for Early Stage Retrieval in Two-stage Ranking. - Haruka Kiyohara; Mihaela Curmei; Ariel Evnine; Shankar Kalyanaraman; Israel Nir; Ana-Roxana Pop; Nitzan Razin; Sarah Dean; Thorsten Joachims; Udi Weinsberg (CoRR, 2026)