Roald Dahl has used numerous rhetorical devices in his poem ‘Television’. In fact, almost every line has something figurative in it, leave alone the overall poetic devices like rhyme, rhythm and meter. Here is a list of all those devices that I have spotted so far —

Simile:
A simile is a direct comparison between two different things using ‘as’ or ‘like’.
In the line “HIS BRAIN BECOMES AS SOFT AS CHEESE”, the poet uses a simile. Brain is compared to cheese for its softness after watching TV.

Metaphor:
“Until they’re absolutely drunk” — Children’s minds, filled with the images and stories of a virtual world, are compared to a drunk man’s imaginary world in an apt metaphor here.

Consonance and Alliteration:
Consonance is repetition of consonant sound in nearby words.
Alliteration is a type of consonance. It is the repetition of consonant sounds in the beginning of nearby words.
Example of alliteration: “And pirates wearing purple pants” / Just How The Camel Got His Hump” 
Example of other consonance: “It makes a child so dull and blind” (repetition of ‘L’ and ‘D’ sounds) / “He can no longer understand” (‘N’ sound repeated)

Assonance:
Assonance is the repetition of vowel sounds in nearby words.s.
“And in its place you can install” (repetition of ‘i‘ sound)

Sibilance:
Again sibilance is also a kind of consonance where ‘S’ sound is repeated making a hissing tone.
For instance, “They sit and stare and stare and sit” / “And wash the dishes in the sink” / “And sailing ships and elephants” etc.

Epizeuxis:
This is the repetition of a single word, with no other words in between.
In line 3, “Is never, NEVER, NEVER let”, the poet has used epizeuxis.

Polysyndeton:
Close repetition of conjunctions. In the poem ‘Television’, polysyndeton is used every here and there.
“They loll and slop and lounge about” / “They sit and stare and stare and sit” / “And treasure isles, and distant shores” / “And sailing ships and elephants” etc.

Enjambment:
Incomplete sentences at the end of lines in poetry.
Enjambment is used all over the poem. For example, the first sentence ends at the 6th line, making the previous lines incomplete.

Synecdoche:
Referring to a part by the whole or whole by a part.
In the line “A dozen eyeballs on the floor” eyeballs refer to children, a part of their body is mentioned to mean the whole.

Allusion:
Allusion is the covert reference to another work of literature or art.
In the poem Television, the poet refers to other authors’ books and characters like Penelope, Squirrel, Pigling, Mrs. Tiggy-Winkle etc. without openly acknowledging them. This is called allusion.

Elision:
Omission of one or more letters in speech, making it colloquial.
“We’ll answer”, “you’ll cry”, “in someone’s place we saw”, “they’re hypnotised” etc.

Anaphora:
Repetition of the same word or set of words in a paragraph.
‘All right!’ you’ll cry. ‘All right!’ you’ll say

Paradox:
Use of apparently contradictory ideas to point out some underlying truth.
I think the line “We’ll answer this by asking you” is paradoxical. At first it seems contradictory that the speaker would ask a question instead of giving an answer for the question asked to him, but the truth is that his answer lies in the question he asks.

Rhetorical question:
A rhetorical question is a question which is not meant for an answer.
Have you forgotten? Don’t you know?

Polyptoton:
Repetition of words derived from the same root.
And in the bedroom, by the bed

Conduplicatio:
Repetition of a key word for giving an emphasis.
“THEY … USED … TO … READ, They’d READ and READ, / AND READ and READ, and then proceed / To READ some more”

Tautology:
Superfluous repetition of the same sense in different words.
“That nauseating, foul, unclean, / Repulsive television screen!” 

(I recommend you checking this Wiki list of all figure of speech for further reference. And if you have found something that I missed, please submit an answer below. Thank you.)