Change checkpoint key used to identify CLIP models in single file checkpoints

[DN6]

What does this PR do?

Some single file checkpoints do not include a position ids key for the CLIP model. This can cause issues loading the CLIP model as we cannot detect it in the checkpoint, such as #8228.

This PR changes the checkpoint key to the position embeds weights key which should always be present in the checkpoint.

Fixes # (issue)

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[vladmandic]

@yiyixuxu @DN6 this may necessitate a service release (0.28.1) given how frequent are those models?

[yiyixuxu]

@vladmandic
does this affect a lot of checkpoints? I did not know that
but yeah we're going to a patch release and will include this fix!