What should I do with the stroller I took from the Grand Mosque in Mecca after losing my own stroller, knowing that my action is considered a sin?

You made a mistake by taking the trolley, as it is not necessarily a found item (luqatah); its owner might have left it temporarily. Your forgetting the ruling on found items in the Haram does not absolve you of responsibility, for it is impermissible to take what does not belong to you. You must repent and return the trolley to the committee responsible for found items in the Haram, as ownership of the trolley does not transfer to you merely because you forgot.