Add tokenizer truncation side option

[jon-tow]

🚀 The feature, motivation, and pitch

Some tasks may benefit from truncating on different sides of batched prompts. We should add a configuration option to handle this from a high level as discussed here. Pasting discussion for quick view:

CC @reciprocated and @PhungVanDuy just in case y'all would like to add any extra comments or context.

Alternatives

No response

Additional context

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[PhungVanDuy]

Thank you for this issue, should we also consider adding the padding side along with the truncation side?