xla

ae35e1e2 - [LTC] Use torch::lazy::Util::ComputePostOrder (#4178)

[LTC] Use torch::lazy::Util::ComputePostOrder (#4178) Summary: This pull request replaces all Util::ComputePostOrder with torch::lazy::Util::ComputePostOrder. In order, it also removes the xla implementation plus GetGraphSize. Test Plan: CI