[torchgen] Relax the restriction on number of custom namespaces (#83580)
Summary:
We started to see use cases where it involves more than 1 custom namespace to live within the same yaml file. Hence relaxing the restriction that 1 yaml file can only have 1 custom namespace other than `aten`. Updated unit test as well.
Differential Revision: D38775685
Pull Request resolved: https://github.com/pytorch/pytorch/pull/83580
Approved by: https://github.com/JacobSzwejbka
