[generate] move SinkCache to a custom_generate repo

[gante]

What does this PR do?

Moves the deprecated SinkCache to a custom_generate repo: https://huggingface.co/transformers-community/sink_cache

Users importing SinkCache will see no issue for now. Users using SinkCache will get a message redirecting them to the Hub repo.

Hub usage example:

# requires `transformers>=4.52.0`
from transformers import AutoModelForCausalLM, AutoTokenizer

# Preparing model, tokenizer, and model inputs
tokenizer = AutoTokenizer.from_pretrained("Qwen/Qwen3-0.6B")
model = AutoModelForCausalLM.from_pretrained("Qwen/Qwen3-0.6B", device_map="auto")
messages = [{"role": "user", "content": "Tell me a story about a cat."}]
text = tokenizer.apply_chat_template(
    messages,
    tokenize=False,
    add_generation_prompt=True,
    enable_thinking=False
)
model_inputs = tokenizer([text], return_tensors="pt").to(model.device)

# Using sink cache
gen_out = model.generate(
    # usual `generate` arguments
    **model_inputs,
    do_sample=False,
    max_new_tokens=100,
    return_dict_in_generate=True,
    # sink cache arguments (default `window_length=256`)
    custom_generate="transformers-community/sink_cache",
    trust_remote_code=True,
)
print(tokenizer.batch_decode(gen_out.sequences, skip_special_tokens=True))
assert "sinkcache" in str(type(gen_out.past_key_values)).lower()
# ['user\nTell me a story about a cat.\nassistant\n<think>\n\n</think>\n\nOnce upon a time, in a cozy village nestled
# between rolling hills and a sparkling lake, there lived a cat named Luna. Luna was small and fluffy, with a curious
# eyes that sparkled with wonder. She had a soft, warm coat that shimmered like the morning sun, and her tail was
# always wagging in playful motions.\n\nOne day, while exploring the village, Luna noticed a curious sight: a young
# boy playing with a ball on the lake. She followed him closely, her heart racing']

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[manueldeprada]

LGTM!