[Bugfix] use blockmanagerv1 for encoder-decoder

[heheda12345]

The default block manager v2 cannot support mllama multi-modal model with text-only request. (can be reproduced by pytest -vs test_mllama.py::test_models). The bug is related to encoder-decoder support of block manager v2. So fallback to v1 for all encoder-decoder models.

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[heheda12345]

@comaniac @KuntaiDu @sroy745 By the way, can you help to take a deeper look of why pytest -vs test_mllama.py::test_models fails now?

[comaniac]

Thanks for catching up this issue. @sroy745 should be the best person to look into this as he is also working on encoder-decoder models.

[ywang96]

LGTM but we should look into why block manager v2 doesn't work with encoder decoder models

[ywang96]

Let's change the wording to the following

Block Manager v2 does not support encoder-decoder models currently. Using Block Manager v1 as fallback...

[comaniac]

Marked as a blocker of #8704

[sroy745]

Thanks for pointing out. I will take a look.