Errors in `gto assign` are made for Labels-env approach

Thanks to @omesser to figuring this out. Since `gto assign` behavior in fact depends on the values for `--assignments-per-version` and `--versions-per-stage` options, the absence of them causes confusion. E.g. you expect `Envs-like` approach, which is the default now in `gto show`, but you get `Labels-like` approach for `assign` now.