Fix missing doc out= for torch.cumprod (#19340)

Fix missing doc out= for torch.cumprod (#19340)

Summary:
Fix #19255 by adding the `out=None` argument for `torch.cumprod` missing [here](https://pytorch.org/docs/master/torch.html#torch.cumprod) also added the docstring for `out` in torch.cumsum which was missing [here](https://pytorch.org/docs/master/torch.html#torch.cumsum)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19340

Differential Revision: D14973931

Pulled By: ezyang

fbshipit-source-id: 232f5c9a606b749d67d068afad151539866fedda