Add assertion for always save nemo add model parallel size

[BoxiangW]

What does this PR do ?

always_save_nemo option is not supported yet to work with model parallel. Added assertions to avoid future confusions.

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[maanug-nv]

Seems like we don't have the ability to save '.nemo' in 2.0 right?

[BoxiangW]

Yes, this was an NVbug about NeMo 1.0. We are not going to save .nemo in 2.0 right now

[maanug-nv]

One last request, can we add a test in test_exp_manager.py to ensure this configuration will fail?

[maanug-nv]

Approving. good to merge if tests pass