If you're doing full gas HIIT in conjunction with an outdoor ride, the first 5-10 miles of a ride could be a warmup. But after the HIIT session there probably won't be enough gas in the tank for anything more than a slow and easy cool down ride home. If I have enough energy for a regular ride in conjunction with HIIT, I wasn't doing the HIIT right.
But I stopped doing HIIT outdoors last year. If I'm putting out full effort, it's potentially dangerous to be gasping for breath, with my head hanging down and trying not to fall from dizziness, while on a road. If you check the GCN training tutorial videos, they usually show two or more partners riding together for outdoor HIIT sessions, for that very reason.
I still do hill repeats and high effort training sessions outdoors, but now I do real HIIT efforts only indoors on the trainer. I also want to be watching my heart rate via my handlebar display. Another thing I don't do on outdoor rides -- I don't even have a simple bike speedometer/odometer on my road bikes now. Phone goes in jersey pocket and I look at it only after stopping. I don't want any distractions on roads booby trapped with distracted drivers.
And I keep HIIT sessions fairly short. I need at least 15 minutes to warm up properly, then the HIIT work itself, and another very easy 15 minute cool down. The whole session doesn't take more than 45 minutes, 2/3 of which is warmup and cool down. Some folks can warm up and cool down quicker, so their HIIT sessions might take only 20-30 minutes.