From Wikipedia:

Most prostitutes were female slaves or freedwomen. The balance of voluntary to forced prostitution can only be guessed at. Privately held slaves were considered property under Roman law, so it was legal for an owner to employ them as prostitutes.

In most circumstances, slave prostitutes could be freely and indiscriminately bought, used and sold. Some were slaves of slave pimps.

Sexual services were routinely offered by the slave attendants in Roman baths (thermae) where both male and female patrons were serviced by both male and female attendants

Sometimes, however, the seller of a female slave attached a ne serva prostituatur clause to the ownership papers to prevent her being forcefully prostituted once sold; if the new owner or any owner thereafter used her as a prostitute she would be freed. This may have been an attempt to conserve what honor was possible for the slave herself, or to remove any possibility of dishonour from the vendor, who might otherwise be thought to have played the part of a pimp, and contravened one of the most fundamental of Roman norms.

Under Hadrian, slaves were protected from being sold to pimps or gladiator schools "unless for good reason". Septimius Severus made protection of slaves from forced prostitution a duty of the urban praetor

Seems like it was common enough that there clauses and later laws about it. ~200 years if you want to say Septimius Severus put an end to it.

https://en.wikipedia.org/wiki/Prostitution_in_ancient_Rome

People did regularly torture their slaves, even if you're not considering the act of slavery itself torture. Do you have a reason to suggest that they did not?

They seem to torture them just because. In pretty much any minority ruled states the majority were/are just generally abused. See South Africa, the US, Iraq, Iran, Saudi Arabia, India (castes), etc etc.