Add new import paths to mnist_tpu.py (tensorflow#4133)

* Add new import paths to mnist_tpu.py

This adds new import paths to mnist_tpu.py so individuals can check out the git repository and run `python mnist_tpu.py` without finagling with python import search paths.

* Fix lint errors

* Add disable= to pylint instructions

* Remove parent path

* Update comments

official/mnist/mnist_tpu.py

@@ -23,10 +23,16 @@
 from __future__ import division
 from __future__ import print_function
 
+import os
+import sys
+
 import tensorflow as tf  # pylint: disable=g-bad-import-order
 
-from official.mnist import dataset
-from official.mnist import mnist
+# For open source environment, add grandparent directory for import
+sys.path.append(os.path.dirname(os.path.dirname(os.path.abspath(sys.path[0]))))
+
+from official.mnist import dataset  # pylint: disable=wrong-import-position
+from official.mnist import mnist  # pylint: disable=wrong-import-position
 
 # Cloud TPU Cluster Resolver flags
 tf.flags.DEFINE_string(