add decay parameter in ref_adagrad (#15329)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15329
Add decay parameter to match with C++ Adagrad implementation.
Reviewed By: chocjy
Differential Revision: D13300991
fbshipit-source-id: db734df0202d8f5fd156f2742207d0b5a3aa7348
