fix TrainerState doc because num_input_tokens_seen is unused by defau…
Author: techkang
Pull request: yes


# What does this PR do?

Fix https://github.com/huggingface/transformers/issues/34567.
"`include_num_input_tokens_seen` argument is set to `False` by default, similar to `load_best_model_at_end`. However, the documentation for `best_metric` and `best_model_checkpoint` strongly implies that these two arguments will have no effect when the corresponding flag is not set. This PR makes the documentation consistent."

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Fixes # (issue)


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Comments:
cc @stevhliu!

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