Easier the modification of ns-train colmap options such as --downscale-factor

[remmel]

Finding how to disable/change downscale using CLI is not easy, while this is as easy as doing: ns-train splatfacto --data mydatadir colmap --downscale-factor 1

The doc indicates the existance of such a parameter

I was expecting to find its presence in ns-train splatfacto --help but this is in ns-train splatfacto colmap --help. The problem is the same for all subcommand options.

Somes ideas:

1. add more information in optional subcommands in ns-train splatfacto --help such as: ns-train splatfacto [subcommand] --help to get pipeline.datamanager.dataparser options
2. add in some place in the documentation
3. move the subcommand help in the main help
4. move the options to --pipeline.datamanager.dataparser.* eg :--pipeline.datamanager.dataparser.downscale-factor
5. Update that message Auto image downscale factor of 4 nerfstudio_dataparser.py:484

At least that information is now here in that github issue and I'm happy with that!