The simple answer is, no, ADHD is not a mild form of autism. As autism is a spectrum disorder, there are milder and more severe classifications.
Autism Spectrum Disorders (ASD) include Asperger's Syndrome/high-functioning autism, autism, Pervasive Developmental Disorder Not Otherwise Specified, and sometimes include Rett syndrome and childhood disintegrative disorder. Where the severe end of the spectrum might be characterized by the autistic savant, with very deep, but very narrow interests and very little typical social interaction; the mild end would be someone with Asperger's Syndrome. Autism is a neural developmental disorder characterized by impaired social interaction and communication, and by restricted and repetitive behavior. Asperger's Syndrome (AS) would further be characterized by the preservation of language and cognitive (learning and thinking) skills.
ADHD and its relation to autism is, though, rather integral. A common misdiagnosis for AS is ADHD. In addition, ADHD is commonly diagnosed comorbid with AS. My son was diagnosed with AS and mild ADHD. There is an interesting paradox for AS people: an intense small group of obsessions and trouble focusing. The difficult side of this can include being too easily distracted and the danger of physically wandering off. The bright side is that the AS mind is going, going, going, thinking of solutions or processing things.