ggml-cpu : size set_rows conversion buffer for all threads (#30454)
* ggml-cpu : size set_rows conversion buffer for all threads
SET_ROWS is planned with n_tasks = 1, but every thread runs it and uses its own row of the F16 -> F32 conversion buffer, so threads > 0 wrote past the work buffer when the graph had another multi-threaded op.
* move set_rows to n_task = n_threads group