Support "eval" worker for Tensorflow distributed learning

[mmourafiq]

No description provided.

[jorgemf]

Relevant: https://www.tensorflow.org/api_docs/python/tf/estimator/train_and_evaluate

The evaluator task is called evaluator:

TF_CONFIG='{
    "cluster": {
        "chief": ["host0:2222"],
        "worker": ["host1:2222", "host2:2222", "host3:2222"],
        "ps": ["host4:2222", "host5:2222"]
    },
    "task": {"type": "evaluator", "index": 0}
}'

[crafet]

hi@jorgemf
if evaluator set index as 0, how about index for other normal workers?
thanks

[jorgemf]

@crafet you have the type to know whether it is a worker, a ps or an evaluator.

[mmourafiq]

similar to #42 , this should be possible in v1.