[LongT5] disable model parallel test
# What does this PR do?

LongT5 doesn't implement the old model parallel logic. This PR disables the model parallel tests for longt5. 
_The documentation is not available anymore as the PR was closed or merged._<|||||>> Thank you for the quick action, @patil-suraj ❤️
> 
> Just to know: we no longer add `parallelize` to new models, right, like what @patrickvonplaten said it's outdated?

Yes, because now any model can be parallelized using the sharded checkpoint and accelerate utils that Sylvain added. cf https://github.com/huggingface/transformers/pull/17341