ggml-cuda: Adding support for unified memory (#8035)

* Adding support for unified memory

* adding again the documentation about unified memory

* refactoring: Moved the unified memory code in the correct location.

* Fixed compilation error when using hipblas

* cleaning up the documentation

* Updating the documentation

Co-authored-by: Johannes Gäßler <johannesg@5d6.de>

* adding one more case where the PR should not be enabled

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Co-authored-by: matteo serva <matteo.serva@gmail.com>
Co-authored-by: Johannes Gäßler <johannesg@5d6.de>