[Single File] Fix loading for LTX 0.9.7 transformer

[DN6]

What does this PR do?

Fix single file loading for LTX 0.9.7 transformer model if a VAE isn't present in the checkpoint. GGUF versions of the checkpoint don't have a VAE.

Fixes # (issue)
#11574

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[nitinmukesh]

Thank you Dhruv.

[a-r-r-o-w]

LGTM!

[nitinmukesh]

Tested, working fine.