BLD: download_models.py respect HTTP Error 429

[codingl2k1]

No description provided.

[gemini-code-assist]

Code Review

This pull request introduces robust HTTP 429 rate-limit handling and retry logic with exponential backoff to the model downloading script, while also reducing parallel connection limits to prevent rate limiting. The review feedback points out a potential issue with clock drift when parsing the 'Retry-After' header and suggests calculating the delay relative to the server's 'Date' header instead of the local system time.

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