Compute accuracy with `targs` when `input` is bs * n_classes.

def accuracy(input:Tensor, targs:Tensor)->Rank0Tensor:
    "Compute accuracy with `targs` when `input` is bs * n_classes."
    n = targs.shape[0]
    input = input.argmax(dim=-1).view(n,-1)
    targs = targs.view(n,-1)
    return (input==targs).float().mean()