Fixing Argument Error in run_nerf.py and adding gitignore file

[vishnukool]

As pointed out in one of the issues. The below argument is duplicated, and causes an error. graf/submodules/nerf_pytorch/run_nerf.py Line 368 in eaa2d6e parser.add_argument("--N_samples", type=int, default=32*32*4, help='batch size (number of random rays per gradient step)')

It should be parser.add_argument ("- N_rand", type = int, default = 32 * 32 * 4, help = 'batch size (number of random rays per gradient step)')