It sounds like you are really, really suffering. I want to echo what newmommdphd said, as I don't think there's any way for it to be heard too often: your child does not have PDD because he was with your family while you did training. For what it's worth, I say this as a physician about to complete the child psychiatry training of my psychiatry residency, who also happens to have a PhD in neuroscience with a developmental focus. If there is any impact of you pursuing medicine on your child during these years, it seems as likely as anything that the primary impact will actually be extremely positive: you will be a very informed mom, who knows the medical systems that you and your child will be navigating. If you complete your training and work as a physician, that will also provide additional income and resources to help your child that most parents of kids with PDD don't have. Finally, it is extremely hard to parent a child with significant mental health issues, including PDD - and the most resilient parents will have periods of burn out. You will need areas of your life that are separate from this, and where you feel successful and competent. There is every reason to think that long term, having a career like medicine will make you better able to be a parent to a high-needs child.
I want to be clear I'm not meaning you have to stay in medicine to be a good parent, of course! But just to point out that the impact on your child is not obviously bad vs good. And, of course, yet again: that PDD is not a result of your training.
I hope that you continue to find this forum useful - this is certainly a place to discuss and find support for this exact type of thing. If there's any way to make it happen, I think seeing a counselor would be extremely useful, as well. (And remember the rule with counselors: if you try one, and after a couple of sessions it's clear it's not the right match, move on and try someone else - don't just give up!)