Final answer:
The godmother to Ernest Hemingway's son, painted by Pablo Picasso, partner of Alice B. Toklas, and a pioneer of Modernist literature is Gertrude Stein.
Step-by-step explanation:
The author in question, who was the godmother to Ernest Hemingway's son, was painted by Pablo Picasso, and was a partner of Alice B. Toklas, as well as a pioneer of Modernist literature, is Gertrude Stein. Gertrude Stein was a central figure in the Parisian art world and hosted a salon that was frequented by many famous writers and artists of the time, including Hemingway and Picasso. She was known for her own works in literature, making significant contributions to Modernist literature with her experimental approach to language and narrative form. Stein's relationship with Alice B. Toklas was also significant, with the two women sharing a life partnership that was both personal and professional.