TFE: Add tfe.SummaryWriter and usage examples

* Revised contrib/summary/summary_ops.py so that multiple summary writers can be created in the same process, each of them writing to a separate logdir.
* Based on the above, the SummaryWriter class supports multiple instances, each holding an independent global_step counter and an independent logdir.
* As the examples in linear_regerssion.py and cart_pole.py show, the SummaryWriter class simplifies user code by
  1) taking care of the registration of unique writer keys,
  2) moving Tensors from GPU to CPU if necessary,
  3) creating an independent global_step tensor,
  4) wrapping around the details of incrementing global_step.

PiperOrigin-RevId: 170495375