For Diana Perez, a medical resident at the Family Health Center of Harlem, the handwritten thank-you note she received from a patient is all the evidence she needs that she has chosen the right training path. Perez helped the patient, a homeless, West African immigrant who has HIV and other chronic conditions, get the medications and care he needed. She also did the paperwork that documented his medical needs for the nonprofit that helped him apply for asylum and secure housing. “I really like whole-person care,” said Perez, 31, who has been based at this New York City health center for most o…