[executorch][emit] Refactor _tensor_spec_to_evalue

^

adding more logic to _tensor_spec_to_evalue in the next diff; simplifying it now.

Otherwise, linter error on complexity.

Differential Revision: [D66847875](https://our.internmc.facebook.com/intern/diff/D66847875/)

ghstack-source-id: 256981105
Pull Request resolved: https://github.com/pytorch/executorch/pull/7233

Co-authored-by: lucylq <lfq@meta.com>