[caffe2] update make_cifar_db to move the string into DB::Put() (#60692)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/60692
Update make_cifar_db.cc to work with the DB API changes in D29204425 (https://github.com/pytorch/pytorch/commit/00896cb9ed931bb816cf1a5e5f13ddc6b229378b).
Test Plan: buck build caffe2/binaries:make_cifar_db
Differential Revision: D29374754
fbshipit-source-id: 23d2acd24031d11071791e398433b537215ffd38
