Fix metavar for --input-size

[JosuaRieder]

The argument parser framework already handles repeating metavars when nargs is set to an integer.

Used to print:

  --input-size N N N N N N N N N
                        Input all image dimensions (d h w, e.g. --input-size 3
                        224 224), uses model default if empty

Now prints:
--input-size N N N Input all image dimensions (d h w, e.g. --input-size 3 224 224), uses model default if empty