With the charming exception of Disney-Pixar’s avian mini-adventure, “Piper,” this year’s Oscar nominees for best animated short film is a mixed bag of tales of whimsy, nostalgia, coming-of-age, tragedy, and personal loss. Whether it’s an existential crisis of a girl in “Blind Vaysha” or the unrequited affection and generosity of a friend in “Pear Cider and Cigarettes,” all nominees offer a personal vision and emotional subtlety that vary in tone, style, subject and inspiration.