Run pmap_test.py with 8 CPU devices. (#1844)
Run pmap_test.py with 8 CPU devices. (#1844)

This lets us test many aspects of our parallel computing
infrastructure without the need for multiple accelerators (it's still
important to test actual multi-accelerator platforms to get full
end-to-end coverage). Using the --xla_force_host_platform_device_count
can also be used to quickly test out or prototype pmap code.
google/jax,tensorflow/probability,google/jax,google/jax,tensorflow/probability,google/jax