🔍 Add guidance on choosing max_length value and include visualization tool

[qgallouedec]

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[HuggingFaceDocBuilderDev]

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[edbeeching]

LGTM with minor comment about mentioning padding free training which should be more efficient with longer seqs + no packinh.

[lewtun]

Super nice visualiser! LGTM with a nit about the viz labels