Add example to WeightedRandomSampler doc string (#17432)
Summary: Example for the weighted random sampler are missing [here](https://pytorch.org/docs/stable/data.html#torch.utils.data.WeightedRandomSampler)
Differential Revision: D14198642
Pulled By: soumith
fbshipit-source-id: af6d8445d31304011002dd4308faaf40b0c1b609
