Add wait-for-model header when sending request to Inference API

[Wauplin]

Should fix #2175.

In the current implementation, InferenceClient sends a request every 1s as long as the model is unavailable (HTTP 503). This can lead users to be rate limited even though they don't consume the API (reported here). This PR adds "X-wait-for-model": "1" as header which tell the server to wait for the model to be loaded before returning a response. This way the client doesn't make calls every X seconds for nothing. This X-wait-for-model header is added only when requesting the serverless Inference API.

EDIT: based on @Narsil's comment, header is added to the request only on the second call. This way, user don't reach the rate limit but we are still able to log a message to tell the user the model is not loaded yet.

cc @Narsil (from private slack thread)

[Narsil]

Oh by the way, for the "stuck" feeling you were mentionning.

What about doing 1 query without the wait, and only adding the way after the first retry ?

2 Queries, but at least with a good error message.

[HuggingFaceDocBuilderDev]

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[Wauplin]

What about doing 1 query without the wait, and only adding the way after the first retry ?

Nice idea! Implemented it in 8ea3f1d

[LysandreJik]

Sounds good!

[Wauplin]

Thanks for the review!