Shape with holes

[LaurentMazare]

This is a quality of life improvement allowing a construction similar to pytorch use of -1 when reshaping a tensor, e.g. t.reshape(5, 2, -1), here we use () instead of -1 so as to still be able to use usize and to ensure at compile time that there is as single hole, so the equivalent construction becomes t.reshape((5, 2, ()))?.