Support providing path to xplane.pb instead of having to provide log-dir

Let's assume I have a xplane.pb file in my current directory. Then in order to load it in tensorboard, I would have to do something like this:

mkdir -p summaries/train_train/plugins/profile/2024_11_15_21_36_34/
mv *.xplane.pb summaries/train_train/plugins/profile/2024_11_15_21_36_34/
tensorboard --logdir summaries/train_train --load_fast=false

It would be really nice if tensorboard had a simple UX like this:

tensorboard --event_file mah-profile.xplane.pb

I tried using --event_file:

tensorboard --event_file gke-tpu-4eba1e59-9k6z.xplane.pb
Error: A logdir or db must be specified. For example `tensorboard --logdir mylogdir` or `tensorboard --db sqlite:~/.tensorboard.db`. Run `tensorboard --helpfull` for details and examples.

Start at the TensorBoard command-line handling for --event_file and the validation that currently requires a logdir or database. Trace how the supplied xplane.pb path is processed, then verify that tensorboard --event_file mah-profile.xplane.pb loads the profile without requiring a log directory.