CMake: Clean up unused definitions (#69216)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69216
Currently `torch_cpu` has command line arguments relating to cuda
libraries e.g. `-DMAGMA_V2`. This happens because
`include_directories` and `add_definitions` indescriminately change
the compile commands of all targets.
Instead creating a proper magma target allows limiting the flags to
just `torch_cuda`.
Test Plan: Imported from OSS
Reviewed By: dagitses
Differential Revision: D33794174
Pulled By: malfet
fbshipit-source-id: 762eabf3b9576bef94e8caa3ed4764c0e2c72b08
(cherry picked from commit f7d127b654330e3b37a134200571122aab08079b)
