set maxlen based on stage and obey max_seq_length settings

[arendu]

What does this PR do ?

memory/tokens for generation is pre-assigned with size min(context_lenght + tokens_to_generate, max_seq_length)

This causes a crash if context_length + tokens_to_generate > max_seq_length because the clipping strategy was not doing any clipping of the maxlen.

This PR adds the clipping strategy for models that don't use learned position embeddings.

Collection: [NLP]

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This PR is stale because it has been open for 14 days with no activity. Remove stale label or comment or update or this will be closed in 7 days.

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This PR was closed because it has been inactive for 7 days since being marked as stale.