[InferenceSnippet] If access token is not provided, default to HF_TOKEN environment variable

[Wauplin]

Originally from @gary149 on slack (private link):

Instead of using dummy values like hf_xxxxxxxxxxxx, it would be good if inference snippets could use the HF_TOKEN env variable by default - if nothing is provided. Would allow to be able to directly copy-paste the snippets and run them if env variable exists.

In practice:

• curl should use $HF_TOKEN
• python should use os.environ["HF_TOKEN"] (⚠ requires import os in code snippet)
• js shoud use process.env.HF_TOKEN

We still want to be able to pass a custom key to getInferenceSnippets and have it work with it (typically for direct calls without routing).
We want it to work with as many clients and tasks as possible but if too difficult to have a 100% coverage, let's focus on the most important ones.

[julien-c]

• js shoud use process.env.HF_TOKEN

well JS can also run in browser or other envs, but maybe not a big deal, people will know how to change it?

[Wauplin]

yep true but still better than token="hf_xxxxxxxxxx" in my opinion