AllReduce strategy crashes on multinode CPU-only cluster

[odp]

Please describe the bug

example/linear_regression.py with AllReduce strategy crashes when run on a CPU-only multinode cluster with the resource spec like:

nodes:
  - address: X.X.X.X
    cpus: [0]
    chief: true
  - address: X.X.X.X
    cpus: [0]
    ssh_config: conf
ssh:
  conf:
    username: XXX
    key_file: YYY.pem
    shared_envs:
      LD_LIBRARY_PATH: '$LD_LIBRARY_PATH:/usr/local/cuda/lib64'

Output

Segmentation fault (core dumped)
2021-02-24 14:39:39.456448: W tensorflow/core/distributed_runtime/rpc/grpc_remote_master.cc:160] RPC failed with status = "Unavailable: Socket closed" and grpc_error_string = "{"created":"@1614195579.456255335","description":"Error received from peer ipv4:127.0.0.1:15000","file":"external/com_github_grpc_grpc/src/core/lib/surface/call.cc","file_line":1056,"grpc_message":"Socket closed","grpc_status":14}", maybe retrying the RPC
Traceback (most recent call last):
  File "/home/xxx/.local/lib/python3.6/site-packages/tensorflow/python/client/session.py", line 1365, in _do_call
    return fn(*args)
  File "/home/xxx/.local/lib/python3.6/site-packages/tensorflow/python/client/session.py", line 1350, in _run_fn
    target_list, run_metadata)
  File "/home/xxx/.local/lib/python3.6/site-packages/tensorflow/python/client/session.py", line 1443, in _call_tf_sessionrun
    run_metadata)
tensorflow.python.framework.errors_impl.UnavailableError: From /job:worker/replica:0/task:1:
Socket closed
Additional GRPC error information:
{"created":"@1614195579.456825677","description":"Error received from peer ipv4:10.20.41.65:15000","file":"external/com_github_grpc_grpc/src/core/lib/surface/call.cc","file_line":1056,"grpc_message":"Socket closed","grpc_status":14}
	 [[{{node scoped_allocator_1_2_CollectiveReduce}}]]

Please describe the expected behavior

System information and environment

• OS Platform and Distribution (e.g., Linux Ubuntu 16.04): 18.04
• TensorFlow version: 2.2.0
• Python version: 3.6.12
• GCC/Compiler version (if compiling from source):
• CUDA version: 10.1
• NCCL version: 10
• cuDNN version: 10.1
• GPU model and memory: GTX 1080 Ti, 12G
• AutoDist version: github master

To Reproduce
Steps to reproduce the behavior:
Run example/linear_regression.py on a multi-node multi-CPU cluster.

Screenshots
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Code snippet to reproduce the problem

Additional information
Add any other context about the problem here or include any logs that would be helpful to diagnose the problem.

Works on a the GPUs of the same cluster.