Correcting the pipeline rpc example

[panchul]

This PR fixes the error in distributed/rpc/pipeline/main.py. Here is how it looks like:

distributed/rpc/pipeline$ python ./main.py
Traceback (most recent call last):
  File "./main.py", line 286, in <module>
    mp.spawn(run_worker, args=(world_size, num_split), nprocs=world_size, join=True)
  File "/anaconda/envs/mypytorchenv_gpu/lib/python3.7/site-packages/torch/multiprocessing/spawn.py", line 200, in spawn
    return start_processes(fn, args, nprocs, join, daemon, start_method='spawn')
  File "/anaconda/envs/mypytorchenv_gpu/lib/python3.7/site-packages/torch/multiprocessing/spawn.py", line 158, in start_processes
    while not context.join():
  File "/anaconda/envs/mypytorchenv_gpu/lib/python3.7/site-packages/torch/multiprocessing/spawn.py", line 119, in join
    raise Exception(msg)
Exception:

-- Process 0 terminated with the following error:
Traceback (most recent call last):
  File "/anaconda/envs/mypytorchenv_gpu/lib/python3.7/site-packages/torch/multiprocessing/spawn.py", line 20, in _wrap
    fn(i, *args)
  File "src/pytorch-examples/distributed/rpc/pipeline/main.py", line 268, in run_worker
    run_master(num_split)
  File "src/pytorch-examples/distributed/rpc/pipeline/main.py", line 231, in run_master
    model = DistResNet50(split_size, ["worker1", "worker2"])
NameError: name 'split_size' is not defined

[soumith]

thank you!