MIKHAIL GORBACHEV'S sweeping proposals for nuclear arms reduction, coming on the heels of George Bush's dramatic proposals two weeks ago to eliminate short-range nuclear weapons, show how seriously the Soviet leader wants his country to become a partner with the West.Reducing dangerous Soviet tactical nuclear weapons and bringing them from their dispersed sites back into areas firmly controlled by Moscow is a major step. Mr. Bush's proposal gave Mr. Gorbachev the opening to take this step, and he did. How quickly or completely Gorbachev can reduce his strategic and tactical nuclear weapons in air, ground, and naval forces is hard to assess. Certainly he is in a much better position to do so than before the attempted coup. Both Gorbachev and Boris Yeltsin, who has new clout with the military, know that economic assistance is linked to good relations with the West. Both know that cutting weapons saves precious money in the long run. Holding the military purse strings at a time of budgetary crisis gives them considerable latitude inside the military. But no one is quite sure how power actually devolves between Gorbachev's crumbling Soviet center, the military, and Mr. Yeltsin. Authority is in a state of constant flux among the three power centers. Soviet disarmament proposals show good faith. But in a technical sense, they do not usher in a nuclear-free era. The truly important long-range missiles that make the Soviet arsenal so potent remain untouched. Nor have the bilateral proposals thus far caused any of the numerous other countries that possess nuclear weapons to offer cuts. The sleeper proposal in Gorbachev's outline is to cease production of weapons-grade fissionable materials. If both sides agree, this plan could create significant pressure on Pakistan, India, and, most important, Israel to show the same restraint.