BANGKOK — Sketching a dire outlook for the Soviet Union, the leader of Moscow's delegation to a world banking meeting warned that major political upheaval could erupt if rebellious republics reject an economic cooperation treaty now under discussion.Grigory Yavlinsky, the radical economist and architect of a plan to keep the Soviet Union intact through economic cooperation, predicted that the beleaguered country had only six months before internal tensions reached the breaking point. Mr. Yavlinsky made his comments at the annual meeting of the World Bank and the International Monetary Fund, which the Soviet Union is attending as an associate member for the first time. Separately, the world's 24 richest countries also pledged to provide financial aid to the breakaway Baltic states of Lithuania, Latvia, and Estonia as well as struggling Albania, which is the newest and poorest member of the IMF. The newly independent Baltic countries are expected to formally join the international financial organization next year. Saying that the Soviet Union is in the midst of a "profound economic crisis," Yavlinsky painted a picture of economic desperation in a report given to leaders of the seven major industrialized countries last weekend and released on Wednesday. The world's economic powers have pledged to assist the Soviet economy but have held off with a definite commitment pending a visit by a financial and economic team, expected in two weeks. In the report, Yavlinsky projected plummeting national output, industrial and agricultural volume, and trade amid rampant inflation and black market activity. The economist, however, insisted the Soviet Union would be able to continue to make payments on its $70 billion foreign debt.