fix the warning message of overflowed sequence
This PR fixes the warning message (of _**PreTrainedTokenizerBase.prepare_for_model**_) when trying to tokenize a model with a sequence that is longer than the specified maximum sequence length.

The current warning message shows the length of the first input_ids, which can be incorrect when a pair is being encoded and the first input length doesn't exceed the limit. The desired warning message should show the overall length of the encoded ids. 