Update CHANGELOG.md for v0.6.3 (backport #473)

[mergify]

This just adds a note about relaxing PyTorch version constraints to our CHANGELOG.md in preparation for releasing v0.6.3.

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This is an automatic backport of pull request #473 done by Mergify.

[mergify]

Cherry-pick of 7d6828e has failed:

On branch mergify/bp/release-v0.6/pr-473
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	both modified:   CHANGELOG.md

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