Gritstone Oncology, the two-year-old Emeryville startup that develops immunotherapy treatments for cancer, has received $93 million in new financing, the company announced Thursday. The privately held company is one of several Bay Area startups making inroads in applying artificial intelligence to drug discovery, a fast-growing segment of the health technology field that has attracted hundreds of millions of dollars from investors over the last few years. Gritstone, though, has amassed more financing than many other drug discovery startups that incorporate AI.