Officer Training: use while attacking to reroll 1 attack dice. Then, if you are a Leader, draw 1 command card.

My first thought, you must reroll the die in order to play the card. But I'm curious if you can choose to reroll zero dice?

There is no "may" or "up to 1". If you can reroll 1 attack die, you must reroll 1 attack die. If all of your attack dice have been rerolled already during this attack, then that part does nothing, and then (if you're a Leader ) you draw 1 command card.

Just to clarify hypotheticals to better understand the finer rules...

As the attacker, you would first declare all your reroll abilities, then do them. Then the opportunity for rerolls as the attacker has come and gone. So I'm thinking it wouldn't be possible to play this card without an attack reroll happening. Is that right?

Weequay Pirate and HK Assassin Droid have attack reroll abilities. If they get another reroll from some other source such as Luke (Hero of the Rebellion), they could've rerolled both attack dice before playing Officer's Training.

Ahsoka can reroll all her attack dice and is a leader

That's what I'm trying to clarify. If the attack rerolls any number of dice, attacker reroll opportunities are done and over with, right? So the attacker now can't play this card.

The way I understand the steps of an attack, you can't see the results of a reroll and then decide to play Officer Training for the card draw.

No.

If you have an ability with trigger X, and X happens, you choose whether to perform the ability. If you do, you return to trigger X and can then resolve other abilities that have trigger X.

See Timing Conflict Resolution from the RRG for abilities with simultaneous triggers. (A summary on page 2: Timing.)

Ability Resolution Order During Attacks - the full story this far

You can thus perform multiple rerolls (of different dice) from different abilities.

What you can't do, is split an ability - if a single ability lets you reroll 1 or 2 dice, you must choose how many and which dice to reroll and then reroll them.