Additional Study Information: This is a Phase I/IIa study, which means that there is some but limited clinical experience in SHR-A1904 (the study drug) clinical trials conducted outside the USA. In this study, the sponsor (Jiangsu Hengrui Pharmaceuticals Co., Ltd.) and study doctors want to learn: How much of the study drug can be given with an acceptable level of side effects. About the safety and tolerability of the study drug. If the bodys immune system has a response to the study drug. The effects of the study drug (good and bad). How much of the study drug is absorbed into the blood and how fast it is removed. If research tests can be used in the future to predict who will benefit from the study drug. Cancers include: colon and rectal cancer, esophageal cancer, lung cancer, pancreatic cancer, and stomach cancer.