Move OS model examples to separate sub-directory

[Phlip79]

What does this PR do ?

As part of #3324, we are re-organizing the examples. This PR moves all OS model examples into a separate sub-directory called open_models.

Also, this PR gets rid of BERT and Mixtral examples. We recommend using Megatron-Bridge when using OS models.

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[Phlip79]

/ok to test 5b97101

[Phlip79]

/ok to test 7c267be

[jaredcasper]

If we support BERT we should have an example of how to run it. If we aren't going to support BERT, we should remove support for it.

What's the point of adding the "open_models" directory, why does it matter?