Only initialize GPU backends if they are not already initialized

PiperOrigin-RevId: 456664792

build/test-requirements.txt

@@ -6,3 +6,4 @@ pillow>=8.3.1,<9.1.0
 pytest-benchmark
 pytest-xdist
 wheel
+numpy<1.23.0

tests/distributed_test.py

@@ -40,6 +40,9 @@ config.parse_flags_with_absl()
 @unittest.skipIf(not portpicker, "Test requires portpicker")
 class DistributedTest(jtu.JaxTestCase):
 
+  # TODO(phawkins): Enable after https://github.com/google/jax/issues/11222
+  # is fixed.
+  @unittest.SkipTest
   def testInitializeAndShutdown(self):
     # Tests the public APIs. Since they use global state, we cannot use
     # concurrency to simulate multiple tasks.