Fix cuFFT plan cache size on CUDA 10 cannot be set to > 4096 (#16384)
Summary:
Doc doesn't need to be changed. Also clarifies two inaccurate comments.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16384
Differential Revision: D13886637
Pulled By: soumith
fbshipit-source-id: 227385008211a6f3ad9135c54fd2d3754cc9daaf
