[BC-Breaking] Rename at::Tensor::base() to _base() (#33316)
Summary:
This PR renames `at::Tensor::base()` to `at::Tensor::_base()`, to achieve parity with Python `torch.Tensor._base` API.
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This PR is BC-breaking in the following way:
Previously, to get the tensor that this tensor is a view of, the user would call `tensor.base()` in C++. Now, they must call `tensor._base()`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33316
Differential Revision: D19905687
Pulled By: yf225
fbshipit-source-id: 949d97b707b2c82becb99ac89e9ac24359d183e6
