message: [XLA:Python] Raise an error if no backends are successfully initialized.
  Prior to this change, this would fail when trying to access list(backends.values())[-1], and give no indication what the actual error was.
diff: @@ -25,6 +25,7 @@ import inspect
  import itertools
  import os
  +from absl import logging
  import numpy as np
  import six
  @@ -213,11 +214,16 @@ def _get_local_backends():
  _local_backends = collections.OrderedDict()
  for name, factory in _local_backend_factories.items():
  + logging.vlog(2, "Initializing backend '%s'" % name)
  try:
  backend = factory()
  except RuntimeError:
  - # If the backend isn't built into the binary, or if it has no devices, we
  - # expect a RuntimeError.
  + if name == 'cpu':
  + # We always expect CPU to initialize successfully.
  + raise
  + else:
  + # If the backend isn't built into the binary, or if it has no devices,
  + # we expect a RuntimeError.
  continue
  _local_backends[name] = backend
  return _local_backends
  