[EZ] Use tokenizer max_seq_len in text_completion

[RdoubleA]

Context

What is the purpose of this PR? Is it to

• add a new feature
• fix a bug
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As titled. We use max_seq_len from the tokenizer in all our datasets instead of an explicit parameter

Test plan

Tested with ds = text_completion_dataset(tokenizer=tokenizer, source="allenai/c4", column="text", data_dir="realnewslike", packed=False, split="train")

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UX

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• I did not change any public API;
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[pytorch-bot]

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ci doesnt pass

[SalmanMohammadi]

Think you gotta move max_seq_len=null from dataset to tokenizer in the Mistral 7B PPO config.