fix: convert back to model descriptor for model in list --downloaded

[reidliu41]

What does this PR do?

[Provide a short summary of what this PR does and why. Link to relevant issues if applicable.]

Currently , model in --downloaded just use the directory(already replace :), so covert back to descriptor keep the same with llama model list, and remove command also use descriptor.

before:
$ llama model list --downloaded
+-------------------------------------+----------+---------------------+
| Model                               | Size     | Modified Time       |
+-------------------------------------+----------+---------------------+
| Llama3.2-1B-Instruct-int4-qlora-eo8 | 1.53 GB  | 2025-02-20 16:32:49 |
+-------------------------------------+----------+---------------------+

after:
$ llama model list --downloaded
+-------------------------------------+----------+---------------------+
| Model                               | Size     | Modified Time       |
+-------------------------------------+----------+---------------------+
| Llama3.2-1B-Instruct:int4-qlora-eo8 | 1.53 GB  | 2025-02-20 16:32:49 |
+-------------------------------------+----------+---------------------+

Test Plan

[Describe the tests you ran to verify your changes with result summaries. Provide clear instructions so the plan can be easily re-executed.]