Fix TunableOp signature generation (#14709)
Currently, all generated op sigs are `TunableOp<ParamsT, TimerT>` which
is causing signature collision when using rocblas solution api. This was
not a problem before `TuningContext`, because the `KernelMap`s were not
shared.
The root cause is the signature is initialized on Op consturction,
specificially, only in base class ctor, which is causing the type info
only caputre base class type info. That is, only the ParamsT + and base
class. After this change, the we will encode the derived class type in
the op sig.
