Skip the Hugging Face token prompt and no-token warning when huggingface_hub already has a cached token

[MaxGhenis]

Problem

download_huggingface_dataset resolves its token only from HUGGING_FACE_TOKEN (or an interactive prompt) through get_or_prompt_hf_token(). huggingface_hub keeps its own token too: huggingface_hub.get_token() returns, in order, a Colab secret, HF_TOKEN / HUGGING_FACE_HUB_TOKEN, or the file written by hf auth login (utils/_auth.py:49 and :118 in huggingface_hub 1.4.1). When core passes token=None, get_token_to_send falls back to that cached token unless HF_HUB_DISABLE_IMPLICIT_TOKEN is set.

Two consequences, both raised as non-blocking findings by the independent reviews of #538 and #540:

1. Prompt (since Send HUGGING_FACE_TOKEN for public but gated Hugging Face repos #538). On a TTY with HUGGING_FACE_TOKEN unset, a developer who already ran hf auth login or set HF_TOKEN is now prompted for a token when downloading a gated repo, as they always were for private repos. Pressing Enter works (core passes token=None and the cached token is used), but the prompt is unnecessary.
2. Warning (Warn when a restricted Hugging Face download resolves no HUGGING_FACE_TOKEN #540). When no HUGGING_FACE_TOKEN resolves, core warns that the download may 401 even if huggingface_hub holds a cached token and the download succeeds.

Proposal

Before prompting or warning, check whether huggingface_hub will send a token anyway:

from huggingface_hub import get_token, constants

implicit_token_available = (
    get_token() is not None and not constants.HF_HUB_DISABLE_IMPLICIT_TOKEN
)

• Skip the getpass prompt when implicit_token_available is true.
• Emit the Warn when a restricted Hugging Face download resolves no HUGGING_FACE_TOKEN #540 warning only when it is false.
• Keep passing token=None in that case, so what goes over the wire is unchanged and core still does not read HF_TOKEN itself (Handle empty HUGGING_FACE_TOKEN gracefully #422).

The #540 reviewer checked that get_token and constants.HF_HUB_DISABLE_IMPLICIT_TOKEN exist from 0.25.1, the current huggingface_hub floor, so this works across the supported range.

Tests to update: the hf-token-only cases in tests/core/tools/test_hugging_face.py currently pin the prompt and the warning on purpose; they would instead assert no prompt and no warning when get_token() returns a token, and keep asserting the old behaviour when it returns None or implicit tokens are disabled.