Are there any mentally retarded people who are also savants, like in autism?

No: Many autistic patients have above average intelligence where as mentally retarded people have below normal intelligence. This lower intelligence will prevent savant like behavior. That does not mean that mr patients cannot make a meaningful contribution to society.

Definitions change: The original term was "idiot savant." the reason these rare individuals are of such interest is that often their overall capabilities, what most of us would think of as intelligence, is quite low, and yet in one or two specific areas, they have skills vastly superior to the average population. The term "autistic savant" is now preferred, but mental retardation usually is a part of the picture.

The current term is : Intellectual disability. Cognitive abilities, both verbal and non-verbal, are fairly evenly below 70, with concomitant social/adaptive delays ; onset before age 18. There are different levels of id; e.g., an iq of 55-69 is the "mildly handicapped" range. 45-65% of people with autistic spectrum disorder also have id. " spikes" in certain areas of ability can be seen in those who have both.