Avoid unnecessary copying data in Saved Variable (#61927)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61927
This is a refactor of `SavedVariable.cpp` to prevent ever defining the `data_` tensor if default hooks are set.
Before the refactor:
```c++
data_ = variable.tensor_data(); // this is wasteful if hooks are defined
register_hooks(Engine::get_default_engine().get_default_saved_variable_hooks());
```
After the refactor:
```c++
if (get_default_hooks_()) {
save_metadata_(variable);
register_hooks_(get_default_hooks_(), variable);
return;
}
save_metadata_(variable);
data_ = variable.tensor_data(); // only needed if hooks are not defined
```
Test Plan: Imported from OSS
Reviewed By: zou3519
Differential Revision: D29848524
Pulled By: Varal7
fbshipit-source-id: abca1eee37a17b47841e28d8a576490913fce1ce
