[PyTorch] Minor improvement in split_with_sizes (#83208)
Don't default-construct a bunch of Tensors only to overwrite them.
Differential Revision: [D38599634](https://our.internmc.facebook.com/intern/diff/D38599634/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/83208
Approved by: https://github.com/albanD
