Use `std::abs` instead of `abs` in lbfgs.cpp (#35698)
Summary:
`abs` is a C-style function that takes only integral argument
`std::abs` is polymorphic and can be applied to both integral and floating point types
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35698
Test Plan: CI
Differential Revision: D20749588
Pulled By: malfet
fbshipit-source-id: b6640af67587650786366fe3907384bc8803069f
