Add PGM support to load_image/save_image
Add PGM support to load_image/save_image


Former-commit-id: 1a4c049e67122d1d43dceb24bf858a6610e470ea
Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide