fix: import ray under a guard (#708)

aphrodite/distributed/device_communicators/tpu_communicator.py

@@ -1,4 +1,3 @@
-import ray
 import torch
 import torch.distributed as dist
 from torch.distributed import ProcessGroup
@@ -6,6 +5,7 @@ from torch.distributed import ProcessGroup
 from aphrodite.platforms import current_platform
 
 if current_platform.is_tpu():
+    import ray
     import torch_xla.core.xla_model as xm
     import torch_xla.runtime as xr
     from torch_xla._internal import pjrt