Improve PNNX robustness

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Improved asset retrieval robustness in Ultralytics exporter.

📊 Key Changes

Added is_url function check to validate URL formats during asset download processes.
Implemented more descriptive logging, including successful asset URL retrievals and warnings when default URLs are used due to retrieval failures.

🎯 Purpose & Impact

Enhances Reliability: Ensures the asset retrieval process is more reliable by verifying URLs.
Increases Transparency: Through better logging, users and developers gain insights into the operational flow, making debugging and tracking easier.
User Experience: These changes aim to streamline the use of the Ultralytics exporter, minimizing potential errors and improving overall user satisfaction. 🚀

Codecov Report

Attention: Patch coverage is 66.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 70.64%. Comparing base (be6bb2a) to head (2e8107c).

:exclamation: Current head 2e8107c differs from pull request most recent head 03e4976. Consider uploading reports for the commit 03e4976 to get more accurate results

Files	Patch %	Lines
ultralytics/engine/exporter.py	66.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #11689 +/- ## =========================================== + Coverage 37.37% 70.64% +33.27% =========================================== Files 122 122 Lines 15590 15591 +1 =========================================== + Hits 5826 11015 +5189 + Misses 9764 4576 -5188 ``` | [Flag](https://app.codecov.io/gh/ultralytics/ultralytics/pull/11689/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ultralytics) | Coverage Δ | | |---|---|---| | [Benchmarks](https://app.codecov.io/gh/ultralytics/ultralytics/pull/11689/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ultralytics) | `35.62% <66.66%> (?)` | | | [GPU](https://app.codecov.io/gh/ultralytics/ultralytics/pull/11689/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ultralytics) | `37.36% <33.33%> (-0.01%)` | :arrow_down: | | [Tests](https://app.codecov.io/gh/ultralytics/ultralytics/pull/11689/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ultralytics) | `66.73% <33.33%> (?)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ultralytics#carryforward-flags-in-the-pull-request-comment) to find out more.

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ultralytics / ultralytics