updated name_inference doc for cumsum and cumprod (#29453)
Summary:
cumsum/cumprod perform their own respective operations over a desired dimension, but there is no reduction in dimensions in the process, i.e. they are not reduction operations and hence just keep the input names of the tensor on which the operation is performed
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29453
Differential Revision: D18455683
Pulled By: anjali411
fbshipit-source-id: 9e250d3077ff3d8f3405d20331f4b6ff05151a28
