“No data” messages are out of date

“No data” messages are out of date

The messages that appear when you select a dashboard for which no data
is loaded are out of date. From a quick skim:

• Some links point to “the TensorBoard tutorial” putatively located at
  https://www.tensorflow.org/get_started/summaries_and_tensorboard,
  but that link now redirects to the general TensorFlow tutorials
  subsite, https://www.tensorflow.org/tutorials.
• Likewise for the “graph visualizer tutorial”.
• The graphs dashboard suggests creating a tf.summary.FileWriter and
  calling its add_graph method; this class does not exist in TF 2.0
  (except via compat.v1), and graph tracing is now done via the
  tf.summary.trace_* functions.
• The profile dashboard’s first tutorial link is broken (404):
  https://colab.research.google.com/github/tensorflow/tensorboard/blob/master/docs/r2/tensorboard_profiling_keras.ipynb
• Beholder.
• The projector dashboard doesn’t have the necessary recommendations
  for TF 2.0 (cf. #2471).
• Some dashboards have no “no-data” help text at all: PR curves, HParams.

Start by locating the no-data help text for the dashboards listed in the issue, then verify each TensorFlow and Colab link and the current TensorFlow 2.0 guidance. Done means outdated instructions and broken links are updated, missing PR curves and HParams help is addressed, and the listed dashboards consistently show accurate no-data messages.