Enhance auth message with API instructions

[noahho]

Updated the authentication message to include API usage instructions to encourage API usage and show way to bypass HF token etc.

Issue

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Motivation and Context

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Public API Changes

• No Public API changes
• Yes, Public API changes (Details below)

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How Has This Been Tested?

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Checklist

• The changes have been tested locally.
• Documentation has been updated (if the public API or usage changes).
• A entry has been added to CHANGELOG.md (if relevant for users).
• The code follows the project's style guidelines.
• I have considered the impact of these changes on the public API.

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[gemini-code-assist]

Code Review

This pull request enhances the authentication error message by adding instructions on how to use the TabPFN API as an alternative to downloading models from Hugging Face. This is a helpful addition for users. My review includes a minor suggestion to improve the formatting of the new message by removing an unnecessary trailing space.

[Copilot]

Pull request overview

This pull request enhances the authentication error message in the model loading functionality by adding a reference to the TabPFN API as an alternative approach to bypass HuggingFace token requirements. The change aims to make users aware that they can use the API instead of dealing with gated model authentication.

• Adds alternative API usage instructions to the authentication error message
• Provides a documentation link to the TabPFN API quickstart guide

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[Copilot]

The error message formatting has inconsistencies and trailing whitespace. Line 266 has a trailing space before the newline character, and the new alternative text lacks proper formatting to match the rest of the message structure. Consider formatting this more consistently with the rest of the message, adding a blank line before "Alternative:" for better readability, and removing the trailing space on line 266.

Suggested change

	"https://docs.priorlabs.ai/how-to-access-gated-models \n"
	"https://docs.priorlabs.ai/how-to-access-gated-models\n"
	"\n"

[klemens-floege]

LGTM!