Who, in 1921, told hemingway that he and his friends who had served in the war "are a lost generation?"?

I believe the answer is:. Gertrude Stein

The term lost generation that mentioned by Gertrude Stein, the great minds that might never show their works in their perspective field due to their participation in the war. Even after they came back safely, their mind is most likely altered in a way that disrupt their creative process.