Tata means bye in Urdu as well as several others languages, and bata is probably pronounced 'beyta' (pronounced like the Greek letter), which is an affectionate term used from an older person to a younger person.

It means just bye bye . Time to leave something or someone

TATA is same as you described but BATA is not. BATA is with no meaning. these words are used with no meaning purely to made a soft and playful sound of the previous word and they are made on the same pattern of the first word like Chaye-Waye, Biscuit-Wiscuit etc I heard someone saying her child " tata bata" it is said to children to motivate them to say tata to others I am not sure though.

Bata is the name of a shoe company. Tata is an Indian company but it is unlikely that the company name is written on a Pakistani Bus. So it may mean "bye". It seems like an advertisement of the Bata company. Have a look at this picture: http://images.city-history.com/1419827702-316--c_e,g_nw,h_0,w_800,f_none,s_1--82b3148ecdca55e4b9018de3ae6072c1cde92dd7--.jpg