Rename capture_pre_autograd_graph private method (#6214)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/6214

These are mainly no-op changes. The underlying function is already changed.

git grep `capture_pre_autograd_graph` finds these helper functions but in fact we already done migrating.

So let's change the name of the method

This is not going to be cherry-picked, but fixed in main.

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Reviewed By: helunwencser

Differential Revision: D64370367

fbshipit-source-id: af7ad4bc34ff8664ab150c70aa2b9a892ecac76c