Revert "[dynamo] Function => FunctionCtx for placeholder obj (#120577)"
This reverts commit ee01d0807b924874a329be78c6ee880f556645db.
Reverted https://github.com/pytorch/pytorch/pull/120577 on behalf of https://github.com/jansel due to Causing breakages internally ([comment](https://github.com/pytorch/pytorch/pull/120577#issuecomment-1970254363))
