Avoid ImportError when importing WebhooksServer and Gradio is not installed

[mariosasko]

Avoids the ImportError when doing from huggingface_hub import * without having Gradio installed.

(The error is now raised when initializing a WebhooksServer object.)

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[Wauplin]

Makes sense, thanks for the PR. Feel free to merge it when the CI is green :)

Out of curiosity, are you using this feature? What's the use case?

[mariosasko]

@Wauplin No, but I use from huggingface_hub import * for debugging (in the REPL) 🙂.

[codecov]

Codecov Report

Patch coverage: 70.00% and project coverage change: -0.02 ⚠️

Comparison is base (b3e21ec) 82.44% compared to head (9c5c958) 82.42%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1482      +/-   ##
==========================================
- Coverage   82.44%   82.42%   -0.02%     
==========================================
  Files          53       53              
  Lines        5707     5708       +1     
==========================================
  Hits         4705     4705              
- Misses       1002     1003       +1     

Impacted Files	Coverage Δ
src/huggingface_hub/_webhooks_server.py	72.54% <66.66%> (-0.93%)	⬇️
src/huggingface_hub/_webhooks_payload.py	100.00% <100.00%> (+1.72%)	⬆️

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