A Phase II Randomized, Controlled Study to Evaluate the Efficacy, Safety, and Pharmacokinetics of HMPL-760 in Combination with R-GemOx versus Placebo in Combination with R-GemOx in Patients with Relapsed/Refractory Diffuse Large B-Cell Lymphoma (R/R DLBCL). The study phases include screening period, treatment period, safety observation period, PFS follow-up period, and OS follow-up period.