[FEATURE]: train seq2seq model

[Sahajtomar]

Describe the feature

Is it not possible yet to train seq2seq model using colossalai? Given scripts are for causalLM models

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Title: [FEATURE]: train seq2seq model

[binmakeswell]

@Sahajtomar This is just an example, welcome to try other models and submit PRs. For example #3567 Thanks.