output_size: the target output size of the image of the form H x W. Can be a tuple (H, W) or a single H for a square image H x H. H and W can be either a int, or None which means the size will be the same as that of the input.
return_indices: if TRUE, will return the indices along with the outputs. Useful to pass to nn_max_unpool2d(). Default: FALSE