Move VariableImpl functions to AutogradMeta and Variable (#15487)
Summary:
In this PR, we are moving all functions away from `Variable::Impl`, in order to get rid of `Variable::Impl` (and the `data_` Tensor in it) in the next PR. Some of the functions (such as `set_requires_grad` / `requires_grad` / `grad`) will be living in `AutogradMeta` class, while others (such as `backward()` / `rebase_history()` / `grad_accumulator()` / `grad_fn()`) will be living in `Variable` class.
This is the 2nd PR mentioned in https://github.com/pytorch/pytorch/issues/13638.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15487
Differential Revision: D13553173
Pulled By: yf225
fbshipit-source-id: 691f9432d0cd0640af380c757f3e3a2f64f8851c
