[mlir][transform] Decouple GPUDeviceMapping attribute from the GPU transfrom dialect code generator

`DeviceMappingAttrInterface` is implemented as unifiying mechanism for thread mapping. A code generator could use any attribute that implements this interface to lower `scf.foreach_thread` to device specific code. It is allowed to choose its own mapping and interpretation.

Currently, GPU transform dialect supports only `GPUThreadMapping` and `GPUBlockMapping`; however, other mappings should to be supported as well. This change addresses this issue. It decouples gpu transform dialect from the `GPUThreadMapping` and `GPUBlockMapping`. Now, they can work any other mapping.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D138020