move test_abs to device-generic tests (#36465)
Summary:
Per title. test_abs used to be marked as slow_test and run on cpu only. Conceptually similar tests are done in TestTorchMathOps, so it's a matter of adding `abs` test there. 2 remaining checks (correct abs for large-valued long tensors, and correct abs for signed zeros) are factored into separate tests.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36465
Differential Revision: D21000248
Pulled By: ngimel
fbshipit-source-id: 8bc8b0da936b1c10fe016ff2f0dbb5ea428e7e61
