projector: always check root logdir for embeddings (#3694)
projector: always check root logdir for embeddings (#3694)

Summary:
Previously, the projector plugin would look for checkpoints in the root
logdir only if there were no other logs. This led to a confusing
discontinuity: if you add logs to a subdirectory of a logdir with only
projector data, suddenly your projector data would disappear in the UI.
This patch changes the projector plugin to _always_ look in the root log
directory for checkpoints. Fixes #3686.

Test Plan:
Three cases to test:

  - checkpoints in root logdir, no other data
  - checkpoints and summary data in root logdir
  - checkpoints in root logdir, summary data in non-root run

Verify that each of these cases lets the projector plugin render and
that the data shows up only once (i.e., it’s not duplicated).

wchargin-branch: projector-always-check-root

tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard