Intellect devourer is not an object
It is an aberration of tiny size.
Nothing states, that they are one and the same entity.
Note, that it is possible to save the host. Furthermore rules state about Intellect devourer leaving the host, when he drops to 0 hp. The host and the Intellect Devourer are two seperate creatures.
How will polymorph work?
While inside a creature, the intellect devourer has total cover against attacks and other effects originating outside its host.
You cannot affect the intellect devourer itself, but only the host.
True polymorph
Choose one creature or nonmagical object that you
can see within range.[...]
The only thing, you will see is once again, the host, not the intellect devourer. Accordingly, to the spell's decription, only one creature will be affected and that will be the host.
What will be the effect?
Despite the fact of being polymorphed, the host is still the same creature.
The effect will be a dog with the intellect devourer inside it's skull. Because True Polymorph usually affects mental ability scores it is possible to notice, that something is wrong, as the dog will have an Intelligence of 12, instead of 3.
Intellect Devourer magically consumes the brain of the victim. Once it leaves, there's just an empty hole where the brain used to be.
To quote the monster description:
the intellect devourer magically consumes the target’s brain
Death Ward, on the other hand, prevents the subject from dropping to 0 HP - it drops to 1 HP instead. So while under DW, the Intellect Devourer is actually protected from ejection, if it indeed gets inside in the first place.
Once it leaves the brain has to be grown back in one round, or the subject dies.
"The body then dies, unless its brain is restored within 1 round."
I was about to write how DW would not protect the victim from dying because of a lack of a brain, but then it struck me that as a DM, I would probably rule that "magically removing someone's brain" would fall into the "effect that would kill it instantly" -category which DW protects the subject from.
Mostly because, while the body is still living under the Intellect Devourer's control, there's nothing of the victim left; no memories, intellect, etc.
(Which opens a whole new can of worms in the form of "brain vs soul" -discussion which is open to many interpretations...)
So how DW works in this case (without finding any official ruling on the matter saying otherwise), in my opinion, would be that the Intellect Devourer could not remove a target's brain and enter it, if the target was under DW's protection (which would then end DW and leave the target open for another try, later on).
Best Answer
The intellect devourer consumes its host's mind in a two step process
First it uses "Devour Intellect"
This ability does not kill the potential host - but it does drop its intelligence to 0 and stun it. This is powerful indeed, but look what the Devourer has to do to accomplish it: have the host fail a save, and then another mechanic which will only go in favor of the Devour half the time against a target of average intelligence. Effectively two failed saves on the same turn. And to do this, it has to be within 10 feet of its host, when it has an AC of 12 and 21 hp (although it does have non-magic damage resistance).
Once the host is stunned (and thus, incapacitated) and has an intelligence of 0 (and thus, -5 on its Intelligence contest) the Devourer can employ its second ability, "Body Thief"
This second ability does kill the host's mind, but it is unlikely to succeed unless the first ability has been used. Remember, to use this, the target must already be incapacitated. The Devourer has an Int mod of +1. Unless its potential host has also had its Int reduced, it doesn't have a great chance to win the Intelligence contest.
Although the Devourer has Multiattack, it cannot use both Devour Intellect and Body Thief on the same turn. So we are talking about a two round process requiring three successful rolls and unable to be initiated unless the Devourer is within 10 feet. This is dangerous against a single opponent who the Devourer manages to surprise. Against an alert party, even one of Level 2, the Devourer is unlikely to be able to finish the second ability, the one that actually kills the host.