Chapter One M-G-M SETS OUT ON A GREAT FILM ADVENTURE FILMING the story of the atom bomb today, as the Atomic Age is still in its infancy and while the atom bomb is contemporary history, marks an event unprecedented in the motion picture industry.   From time to time, pictures of outstanding significance have come from Hollywood visually to entertain and enlighten film audiences throughout the world. But, their subjects have been ripened by age, tested by the judgement of time.   It was inevitable that the story of the atom bomb should one day reach the screen. But, today it marks the stoutest challenge ever placed upon Hollywood's film making ingenuity. It is the human story of half a million Americans, little men and big men, all doing their job in the biggest, most complex task of all time.   From the moment an atom bomb dropped on Hiroshima, on August 6, 1945, it became the greatest news story ever to break upon the consciousness of the civilized world. To this day, with discussions raging everywhere as to its possible influence on the world's future... if any... it remains a vital topic to every living person.