Don't initialize global threadpool in GraphRunner.

TF_Graph creates a ShapeRefiner, which in
turn creates a GraphRunner, which prior to this change would eventually create a
LocalDevice that initialized the global eigen threadpool. This prevents
users from specifying a custom number of threads for the pool via a
ConfigProto.

This change introduces a new device class, SingleThreadedCpuDevice, that can
be used for light-weight computations without initializing the threadpool.

Addresses #18300.

PiperOrigin-RevId: 192188031