Reverse NDRange support

There comes one special case which requires a reverse loop, so I used “ti.ndrange(8,0)” instead. However, the reverse loop just didn’t work with zero warnings or errors (I have no idea whether ti.ndrange can be used to loop in reverse order at that time) Perhaps a warning message can be given when n1 > n2 in ti.ndrange((n1,n2))

import taichi as ti
ti.init(arch=ti.gpu, debug=True, default_fp=ti.f64)

x = ti.field(ti.f64, shape=(10))

@ti.kernel
def main():
    x[9] = 1.0
    for i in ti.ndrange((8,0)):
        x[i] = 2.0 * x[i+1]
        print(x[i])    #print nothing

main()