Fix fake_initialize_model_parallel for MoE models
State: closed | Pull request: yes
Labels: name: NLP

name: Run CICD

# What does this PR do ?

Make the use of megatron RankGenerator consistent with recent megatron changes [!1940](https://github.com/NVIDIA/Megatron-LM/commit/7f22e210cddc3215adda25d9e16ea512dc32458c)

**Collection**: [Note which collection this PR will affect]

# Changelog 
Make the use of RankGenerator consistent with recent Mcore changes [!1940](https://github.com/NVIDIA/Megatron-LM/commit/7f22e210cddc3215adda25d9e16ea512dc32458c)
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

```python
# Add a code snippet demonstrating how to use this 
```

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# Additional Information
* Related to # (issue)


@akoumpa could you trigger CI

<!-- pylint-output-strict-mode-false -->

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

---

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)

-----------------------------------
Your code has been rated at 9.78/10
```

---

Thank you for improving NeMo's documentation!

Closing this since [!11427 ](https://github.com/NVIDIA/NeMo/pull/11427) is merged