Fix PR curve plugin to display correct thresholds

[dcower]

• Motivation for features / changes:

  • The thresholds generated and shown in the PR curve plugin are incorrect (AFAICT) compared to the thresholds used in the PR curve summary.op.
  • The thresholds used in summary.op are:
    [0.0, 1/(num_thresholds-1), 2/(num_thresholds-1), ..., 1.0]
    ... but prior to this change, the thresholds generated in _make_pr_entry and displayed in TensorBoard are:
    [1/num_thresholds, 2/num_thresholds, ..., 1.0].

• Technical description of changes:

  • Modified _make_pr_entry to instead generate thresholds like sumamry.op, as described above.
  • Cleaned up the code in _make_pr_entry to be a bit easier to read / check for correctness.
  • Added tests for summary.op that verify the thresholds used are indeed what are described above.
  • Changed the number of thresholds used in pr_curve_demo from 50 to 51, since we're now including 0 in the thresholds.

• Screenshots of UI changes: N/A (UI is unchanged)

• Detailed steps to verify changes work correctly (as executed by you):

  • Ran TensorBoard with PR curve demo data and verified thresholds are displayed correctly.
  • Ran all tests in pr_curve directory.

• Alternate designs / implementations considered: N/A

[stephanwlee]

Assigning to Nick who, amongst our team member, is the most knowledgeable person on PR curve. While the description sounds correct, I may not know about the historical background of the plugin.

[nfelt]

Thank you very much for this fix, and for cleaning up the code and adding tests as well! A bit embarrassing this was overlooked for so long 😅 but glad it's correct now.

[dcower]

No problem, and thank you for the review! It took me a while to even tell anything was off, so I can definitely understand why it was overlooked 😄