from huggingface_hub import hf_hub_download
hf_hub_download(repo_id="bigscience/T0_3B", filename="config.json", cache_dir="./your/path/bigscience_t0")

Once your file is downloaded and locally cached, specify it's local path to load and use it:

from transformers import AutoConfig
config = AutoConfig.from_pretrained("./your/path/bigscience_t0/config.json")

See the How to download files from the Hub section for more details on downloading files stored on the Hub.