There is a chicken-and-egg problem with nameservers. The registry-registrar protocol (well, EPP nowadays) permits a registrar to configure a domain by indicating which nameserver name is the nameserver for a given domain, but the domain configuration does not include the nameserver IP. It is possible to fake this by putting the nameserver in another domain that has other nameservers, but somewhere someone has to provide an actual IP to start with, so faking just pushes the problem farther away and slows down resolution of your IPs.
So, there is another object in the registry-registrar protocol that is the nameserver, with its IP(s). When the registry's DNS servers are asked for your domain's name server name, they then also provide additional "glue" records with the nameserver's IP, and that solves the chicken-and-egg problem. Those nameservers are searchable in whois (whois is basically a search interface to registry or registrar information).
If your registrar is well coded, it should not let you define a nameserver for your domain that is not actually defined as a nameserver at its registry, but even if your registrar did that it is hard to deal with a nameserver in another TLD which is deleted or whose domain expires.
So, if the nameservers for your domain are in your domain, you should declare their IPs with your registrar who must declare them with the registry, and if the nameservers for your domain are in another domain then the owner of that domain should do it.
Source: I ran a registrar for five years and implemented RRP and EPP clients for different registries.