If your parents insist that they want to start potty training, it’s alright to do so, but everyone must be consistent. Whatever is done at the grandparent’s house must be done at home too.
It’s also important that you talk to your parents and let them know what your beliefs are regarding potty training. Make sure they know that they should avoid any type of negative input to your child. Never scold or punish them for having an accident, or never say anything that will make them feel bad about it. And, if you decide to use a reward system, make sure that is also consistent between the homes.