Some memory saving in large unittests (#90148)
Two tests test_large_cumsum, test_large_cumprod use a lot of memory. This PR:
* Reduces their memory usage by: avoid `self.assertEqual` and avoid a temporary python variable
* Mark their memory requirement by decorator.
related to https://github.com/pytorch/pytorch/issues/84944
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90148
Approved by: https://github.com/soumith
