The qualification for Ph.D. students is B.Sc. and M.Sc in the relevant fields. The coursework mandated for students with these two qualifications is of course different. If you have a degree in a different (science/engineering) stream, you can still do a Ph.D. in other fields if you are selected through the Admissions process. The coursework you will be required to do at TIFR will be decided when you are admitted, depending on your coursework during the degree you enter TIFR with.