Summary:
Implements a backprop-safe version of `torch.acos` that linearly extrapolates the function outside bounds.
Below is a plot of the extrapolated acos for different bounds:
{F611339485}
Reviewed By: bottler, nikhilaravi
Differential Revision: D27945714
fbshipit-source-id: fa2e2385b56d6fe534338d5192447c4a3aec540c
