what is equivalent API in Cuda 7.5 for cutLoadPGM and cutSavePGM ?
cutLoadPGM and cutSavePGM have never been a part of CUDA. They may have been (in the past), part of a collection of utility functions used to keep examples codes that ship with CUDA focused on the essential elements of each example. NVIDIA has always been very clear that such utility functions are not part of CUDA proper, could disappear or materially change at any time, and should therefore not be used by CUDA programmers in their own code.
The author of the code you are looking at apparently did not heed those warnings. If you poke around the internet, you may be able to find a copy of the cutil.cpp file these likely were part of.