Fix fake_initialize_model_parallel for MoE models

[guyueh1]

What does this PR do ?

Make the use of megatron RankGenerator consistent with recent megatron changes !1940

Collection: [Note which collection this PR will affect]

Changelog

Make the use of RankGenerator consistent with recent Mcore changes !1940
In fake_initialize_model_parallel function in nemo/collections/nlp/modules/common/megatron/megatron_init.py

• use ep=1 for decoder_rank_generator, making it treat EP as part of DP
• define a new expert_decoder_rank_generator to handle EP groups/ranks only

Usage

• You can potentially add a usage example below

# Add a code snippet demonstrating how to use this 

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Additional Information

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[guyueh1]

@akoumpa could you trigger CI

[github-actions]

beep boop 🤖: 🙏 The following files have warnings. In case you are familiar with these, please try helping us to improve the code base.

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Your code was analyzed with PyLint. The following annotations have been identified:

************* Module nemo.collections.nlp.modules.common.megatron.megatron_init
nemo/collections/nlp/modules/common/megatron/megatron_init.py:263:0: C0301: Line too long (122/119) (line-too-long)
nemo/collections/nlp/modules/common/megatron/megatron_init.py:264:0: C0301: Line too long (133/119) (line-too-long)
nemo/collections/nlp/modules/common/megatron/megatron_init.py:265:0: C0301: Line too long (121/119) (line-too-long)
nemo/collections/nlp/modules/common/megatron/megatron_init.py:368:0: C0301: Line too long (166/119) (line-too-long)
nemo/collections/nlp/modules/common/megatron/megatron_init.py:88:0: C0116: Missing function or method docstring (missing-function-docstring)

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Your code has been rated at 9.78/10

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Thank you for improving NeMo's documentation!

[guyueh1]

Closing this since !11427 is merged