Update TensorBoard to import pywrap_tensorflow from tensorflow.python

[annarev]

• Motivation for features / changes

Update TensorBoard to import pywrap_tensorflow from tensorflow.python instead of tensorflow.compat.v1. We are removing pywrap_tensorflow from TF API including tf.compat.v1. Importing from tensorflow.python is a temporary solution until a better API for reading records is available for TensorBoard to use.

• Technical description of changes

tensorboard/compat/init.py now includes a function _pywrap_tensorflow to access either actual or stub pywrap_tensorflow implementation.

• Detailed steps to verify changes work correctly (as executed by you)
  bazel test //...
  I verified that the only tests that fail also fail on master without my change.

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[googlebot]

CLAs look good, thanks!

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[nfelt]

LGTM, thanks for the fix and for taking the time to fix the TF stub logic as well! :)