New York University formally notified the union representing its graduate students yesterday that it would no longer bargain with it. In June, the university said it was moving toward severing its relationship with the five-year-old union when its contract expires on Aug. 31. In a memorandum distributed late yesterday to N.Y.U. students and faculty and in a letter to the union, university officials said they decided not to negotiate a new contract. Jacob J. Lew, N.Y.U.'s executive vice president, said in the memo that grievances filed by the union over issues like who would be assigned as graduate teaching assistants...