Use `std::abs` instead of `abs` in lbfgs.cpp (#35974)
Summary:
This supersedes https://github.com/pytorch/pytorch/pull/35698.
`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
This PR also increases `kBatchSize` in `test_optimizer_xor` function in `test/cpp/api/optim.cpp` to fix `OptimTest.XORConvergence_LBFGS` failure under ASAN.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35974
Test Plan: CI
Reviewed By: pbelevich
Differential Revision: D20853570
Pulled By: yf225
fbshipit-source-id: 6135588df2426c5b974e4e097b416955d1907bd4
