Fix KeyError recently introduced in cloud_tpu_init.py

This fixes a bug introduced in jax-ml#24889

Author: skye

jax/_src/cloud_tpu_init.py

@@ -80,7 +80,7 @@ def cloud_tpu_init() -> None:
   os.environ.setdefault('TPU_ML_PLATFORM', 'JAX')
   os.environ.setdefault('TPU_ML_PLATFORM_VERSION', version.__version__)
   os.environ.setdefault('ENABLE_RUNTIME_UPTIME_TELEMETRY', '1')
-  if '--xla_tpu_use_enhanced_launch_barrier' not in os.environ['LIBTPU_INIT_ARGS']:
+  if '--xla_tpu_use_enhanced_launch_barrier' not in os.environ.get('LIBTPU_INIT_ARGS', ''):
     os.environ['LIBTPU_INIT_ARGS'] = os.environ.get('LIBTPU_INIT_ARGS','') + ' --xla_tpu_use_enhanced_launch_barrier=true'
 
   # this makes tensorstore serialization work better on TPU