Add Seq2Seq example using T5

[DEVANSH-GAJJAR]

• Added examples/seq2seq_quickstart.py, a minimal working example using the T5 model for Seq2Seq tasks.
• Demonstrates how to fine-tune a model on a small English-to-French translation dataset.
• Helps beginners quickly understand how to use simpletransformers.Seq2SeqModel.

[DEVANSH-GAJJAR]

Hello Sir,
i have made this PR , and also there are no conflicts with the base branch , kindly consider this PR to get merged

thank you