[Static Runtime] Separate function for getting always_alive values (#61506)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61506
Separate out the logic of GetAlwaysAliveValues from GetLivenessMap so to simplify the code structure. Also you don't need to run GetLivenessMap if optimize_memory is turned off.
Reviewed By: ajyu
Differential Revision: D29423534
fbshipit-source-id: dbdeeb10f7bcad86a24aa12f741f7c9ab946bb3b
