[EZ] Fix PACK_TYPE import in collate

[RdoubleA]

Context

What is the purpose of this PR? Is it to

• add a new feature
• fix a bug
• update tests and/or documentation
• other (please add here)

This should be imported from torchtune.data._common, not torchtune.datasets._packed.

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What are the changes made in this PR?
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As of commit 37c3c79 with merge base 17f0bff ():
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