Discovered in #1721
__xrange()__ was removed in Python 3 in favor of an improved version of __range()__. This PR ensures equivalent functionality in both Python 2 and Python 3.
```
./samples/contrib/ibm-samples/watson/source/model-source-code/tf-model/input_data.py💯40: F821 undefined name 'xrange'
fake_image = [1.0 for _ in xrange(784)]
^
./samples/contrib/ibm-samples/watson/source/model-source-code/tf-model/input_data.py:102:41: F821 undefined name 'xrange'
return [fake_image for _ in xrange(batch_size)], [
^
./samples/contrib/ibm-samples/watson/source/model-source-code/tf-model/input_data.py:103:37: F821 undefined name 'xrange'
fake_label for _ in xrange(batch_size)]
^
```
@gaoning777 @Ark-kun Your reviews please.