National Zoo (Saturday, Panda Fund) - Hundreds of visitors came to bid farewell to Tai Shan at the Smithsonian National Zoo today.     Next week, Tai Shan will be leaving the Zoo for China on Feb. 4.  Tai Shan, the 4-and-a-half-year-old male panda star born at the National Zoo, and Mei Lan, a 3-year-old female panda born at Zoo Atlanta, will travel together onboard a custom-decaled FedEx Express 777 Freighter (777F) - known as the "FedEx Panda Express." Trained since birth to enter small spaces like the crate which will carry him to China, Tai Shan will head to Dulles International Airport on the morning of Feb. 4 and fly directly to Chengdu, China.      His traveling companions will be a zoo keeper and veterinarian familiar to him since his stay at the National Zoo. Tai Shan's panda traveling buddy will be Mei Lan, the three-year-old female panda born at Zoo Atlanta in Georgia.