Add default GpuKernelBase.get_params() that retrieves GPU context
from first node input type, and then factorize get_params() for all
sub-classes that retrieve context by this way.

Method get_params() must be overridden for all subclasses that get
context by different way.

Extend GpuKernelBase interface to allow sub-classes
to use parameters types other than gpu_context_type
(especially ParamsType).