Rename isTensor api -> isCompleteTensor (#18437)
Summary:
Is Tensor has been brought up as misleading a couple times, rename it isCompleteTensor for clarity.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18437
Differential Revision: D14605223
Pulled By: eellison
fbshipit-source-id: 189f67f12cbecd76516a04e67d8145c260c79036
