Make tril_ and triu_ actually in-place (#17031)
Summary:
Currently, when the input tensor `self` is not contiguous, `tril_` and `triu_` calls `self = self.contiguous()`, which allocates a new contiguous tensor and assign it to `self`. This effectively changes the input tensor `self`'s pointer and will break downstream code after Variable/Tensor merge.
This PR fixes it so that `tril_` and `triu_` always update the input tensor in-place and preserve the input tensor's TensorImpl.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17031
Differential Revision: D14069592
Pulled By: yf225
fbshipit-source-id: d188218f426446a44ccc1d33fc28ac3f828c6a05
