A randomized, open-label phase 2 study of denintuzumab mafodotin (SGN-CD19A) plus rituximab, ifosfamide, carboplatin, and etoposide (19A+RICE) chemotherapy vs. RICE in the treatment of patients with relapsed or refractory diffuse large B-cell lymphoma (DLBCL) who are candidates for autologous stem cell transplant