message: [XLA:Python] Cache the backend in xla_client_test.
  This is in preparation for removing backend caching logic from xla_client.
diff: @@ -2029,8 +2029,11 @@ def TestFactory(xla_backend, cloud_tpu=False):
  return tests
  -def InstantiateTests(globals_dict, backend, test_prefix="", **kw):
  - for klass in TestFactory(backend, **kw):
  +def InstantiateTests(globals_dict, backend_fn, test_prefix="", **kw):
  + # Avoid creating a new backend per test (this causes GPU OOM, and is probably
  + # inefficient).
  + backend_fn = functools.lru_cache(maxsize=None)(backend_fn)
  + for klass in TestFactory(backend_fn, **kw):
  test = type(test_prefix + klass.__name__, (klass,), {})
  # Clean up the qualified names of the tests to not include the test factory.
  test.__qualname__ = test.__name__
  