3rd party XLA devices

This directory is intended as a place for 3rd party XLA devices which are not integrated into the public repository.

By adding entries to the BUILD target in this directory, a third party device can be included as a dependency of the JIT subsystem.

For integration into the unit test system, see the files:

• tensorflow/compiler/tests/plugin.bzl
• tensorflow/compiler/xla/tests/plugin.bzl