docs(samples): Added seq2seq sample

[ivanmkc]

Adds seq2seq sample for forecasting.

Fixes b/241717026

[snippet-bot]

Here is the summary of changes.

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• samples/model-builder/create_training_pipeline_forecasting_seq2seq_sample.py:20, tag aiplatform_sdk_create_training_pipeline_forecasting_seq2seq_sample

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