py bindings -- add more usage help and unbiased timings to autotuner
py bindings -- add more usage help and unbiased timings to autotuner


Former-commit-id: 625afbe7468be5cdfb34f9590467503ff1ca9f95
Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide