Soviet President Andrei A. Gromyko, reported to be hospitalized with a high fever, is back at work, according to the Soviet news media. Reports of his ill health were fueled when he failed to attend last week's Kremlin ceremony commemorating the 116th anniversary of the birth of Soviet founder V.I. Lenin. Reports in the party newspaper Pravda, and from the official news agency Tass, said Gromyko, 76, attended Friday's meeting of the Presidium of the Supreme Soviet. Gromyko who served as foreign minister for 28 years before becoming Soviet president last July, is believed to have coronary problems.