For example, for DistilBERT we have:
thon

from transformers.models.distilbert import DistilBertConfig, DistilBertOnnxConfig
config = DistilBertConfig()
onnx_config = DistilBertOnnxConfig(config)
print(list(onnx_config.outputs.keys()))
["last_hidden_state"]

The process is identical for TensorFlow checkpoints on the Hub.