Add activation/weight histograms for CIFAR-10

[dustinvtran]

@dustinvtran
For some reason, naively turning it on didn't work for me in variational_inference.py.

  tensorboard_cb = tf.keras.callbacks.TensorBoard(log_dir=FLAGS.output_dir,
                                                  # TODO(trandustin): This
                                                  # doesn't work(?).
                                                  # histogram_freq=5,
                                                  write_graph=False)

google/edward2#73

[dustinvtran]

Looking at this issue, I'm not sure we do want histograms by default. They're fairly intensive to compute (I think?).

[dustinvtran]

Marking resolved. It's too expensive as a default.