Technically, a dead human is not a human
No, technically a dead human is indeed a human, albeit a dead one.
and we give the attribute of walking to the dead. So, Is "the walking dead" a personification?
First off, there is nothing here about being human. The words we are working with are the, walking and dead. If the apple on my desk grew legs and walked, I could reasonably refer to it as the walking dead, but not as human.
Some stuff in the spoiler that I won't quote as it's in a spoiler.
The first thing to bear in mind here, is that these beings do not exist. Now that might seem obvious, but it does have a significant point: The explanation given for "walking dead" in several different fictions about them differs. In some the explanation is purely supernatural, in some there is an infection of some sort (whether explained as a variant of some sort of real-world microbe like viruses and bacteria, or not) that affects living people, and makes them behave in that way, in some a similar sort of infection takes place upon brain death, and in some there is another explanation given, or none at all.
Because of this, we can't take the explanation of what the "walking dead" are in either Kirkman's work, or Darabont's adaptation, as self-sufficient in answering a question about how the phrase is used when talking about all such stories.
Now, as such, in some of these cases "walking dead" is quite literally what they are—they are dead, and they walk—and in others it is in fact "dead" that is being used figuratively, as they are still alive.
Now, outside of the diagesis of any particular story, we have the expressions "living dead" and "walking dead" that are applied to all such stories.
Of these, "living dead" was the earlier, and it is an apparent oxymoron, yet not a nonsensical one: the beings so-described have features both of the dead and of the living, and so the apparent oxymoron description in fact serves well. The contradiction also served another purpose, which was to induce a sense of horror (that is, a mixture of fear and revulsion) in combining a word that we are conditioned to have positive associations with and one that we are conditioned to have negative associations with.
One of the things about using an oxymoron for effect though, is that it gets tired after a while. In 1968 it that oxymoron gave it an evocative power that by the end of the 80s (when there was a glut in such films, many of them of notoriously poor quality) it had lost, and so the contradiction began to sound silly rather than horrific.
For that reason, the writers of such fictions began to search for other terms. They'd already abused zombie (which I'll talk about in more detail in a minute), but other terms were also used. "Walking dead" has the advantage of neither having the tenuous link to beliefs about zombies that were a problem with zombie (particularly among readers who were aware of the term's history, and the more well-read readers of horror would be particularly likely to know about that), and the fact that it is quite directly and simply applied; the walking dead are dead, and they walk, simple!
There is though a figurative use as well, in that "walking dead" has had earlier uses, of a live person who is doomed, due to illness, sentence of execution, threat made by the speaker ("you're dead!"), or being party to a military action unlikely to succeed. In the case of Kirkman's story, Rick Grimes, Maggie Greene, etc. are all "the walking dead" in this sense that their survival seems very unlikely.
Calling the "living" characters "the walking dead" is a form of prolepsis (not the only definition for that term as applied to rhetoric), referring to what they will inevitably be, as if they already are.
Using "The Walking Dead" as the title of a story where the term applies both literally to one part of the story (the "walkers") and figuratively to another part (the protagonists) is a case of dramatic irony.
Also, can we say that "the walking dead" is an antonomasia (a kind of metonymy) for zombie?
Certainly not antonomasia. A case of metonymy certainly, though it's the other way around. To look at this we need to look a bit more at the history of 19th & 20th century horror.
Zombie (zonbi, zombi, etc.) refers to a concept in a set of folklore and religious beliefs, particularly related to Voudoun, Yoruba, Candomblé, Lucumi, Macumba, Santeria, and similar religions. Note that I say "related to" rather than "part of", because just as there is a link but not a complete coincidence between e.g. European folklore about "dressing trees" for a blessing and the religion (generally Catholicism) of those who practice them, so too there might be beliefs found among some who believe in Voudoun that wouldn't be as common among trained Vodouisants: Here we're talking about the entire complicated mess of what different people from a bunch of different—though related—religions with different degrees of education in that religion, believe in with different degrees of sincerity (just as many European people will know and respect the fairy-lore of their country without actually believing in fairies).
As such, we can't really say any particular idea about zonbi is "the real", though a given practitioner might hold that one version is. (That one Vodouisante of my acquaintance has a love of 21st Century zombie-apocalypse stories only makes things more complicated.)
As such, zombie originally applied to a bunch of different beliefs, which are still evolving among the communities that belief or believed in them.
Now, outsiders have been using the idea of the zombie since at least the mid-19th Century ("The Unknown Painter"/"A Story of Murillo's Pupil"/"Murillo and his Slave"), with zombie films being made in the early 20th Century (White Zombie, 1932, I Walked with a Zombie, 1943) though rare later (The Serpent and the Rainbow, 1988 being an exception, but a rather tenuous one in a few ways), partly because the more recent Walking Dead genre so altered the global-north's idea of what "Zombie" means and partly because the position of the genre in the history of race relations in America and the Caribbean is complicated to say the least.
The modern "Walking Dead"/"Living Dead" film that reinvented the zombie as understood in the global north started with Romero's Night of the Living Dead (1968). Now, at no point in that film is the word "zombie", "zonbi" or anything similar used.
In fact, the predecessor of the Walking Dead in such stories is not the Zombie at all, but the Vampire.
Vampire stories have themselves undergone an evolution and thanks largely to the 19th Century Dubliners Bram Stoker (Dracula, 1897) and Sheridan Le Fanu (Carmilla, 1871; much superior to Dracula in many ways IMO and sadly much less well-known), though also influenced by Varney the Vampire; or, the Feast of Blood (1845-47), the idea of the vampire as an aristocratic figure emerged with these works and then evolved further into that aristocratic stance with the remaining elements of mindless hunger becoming less and less pronounced.
Richard Matheson's I Am Legend (1954, ironically, now often thought of as a "zombie story" too) harked to the abandoned elements of the vampire story, and Romero harked to that and other elements of European folklore concerned with vampires, revenants and other European folklore about beings that had features of both the dead and the living.
Hence the "living dead"/"walking dead" is an invention of Romero's, based on earlier European and North American stories, and European folklore.
It was only some time after that, that "zombie" was used to describe these creatures, though Romero himself used the term in his later films.
As such, "zombie" is a sort of metonym for the walking dead; taking one example of a creature held to exhibit features of both the dead and the living, and the applying its name to other such creatures after the fact.
Some later horror stories have re-examined this connection and either abandoned it ("The Walking Dead" being an example in not using the term, setting a trend that would later become common in e.g. "The Infected" in 28 Days Later and "Partially Deceased Syndrome"/"Rotters" in In the Flesh), some have tried to re-combine elements of Voudoun and similar beliefs with the European-based Living Dead (e.g. the Anita Blake Vampire Hunter series) and some have abandoned the 20th Century Living Dead to examine Hiatian beliefs and outsiders reactions to it (e.g. "Bitter Grounds").
"The Walking Dead" isn't a metotym, as it applies literally to all the creatures it is used for. Indeed, it is used only for a subset of them, as it could reasonably be used of vampires, revenants, and just about any other idea of reanimated corpses (except perhaps the jiang shi as they're the hopping dead), but it isn't.