Mixture of Expert with Expert Parallel -- initialization in nemo doesn't have expert_model_parallel flag
**Describe the bug**

The model parallel initialisation in NeMo doesn't account for `expert_model_parallel_size` in `megatron_base_model.py` which is currently supported in the latest version of Megatron-LM.

```
        initialize_model_parallel_for_nemo(
            world_size=init_world_size,
            global_rank=init_global_rank,
            local_rank=init_local_rank,
            tensor_model_parallel_size=cfg.get('tensor_model_parallel_size', 1),
            pipeline_model_parallel_size=cfg.get('pipeline_model_parallel_size', 1),
            virtual_pipeline_model_parallel_size=vp_size,
            pipeline_model_parallel_split_rank=cfg.get('pipeline_model_parallel_split_rank', 0),
            micro_batch_size=cfg.get('micro_batch_size'),
            global_batch_size=cfg.get('global_batch_size'),
            rampup_batch_size=cfg.get('rampup_batch_size', None),
            use_fp8=cfg.get('fp8', False),
            init_mpi_proc_group=cfg.get('ub_tp_comm_overlap', False),
            seed=self.cfg.get('seed', 1234),
            apex_transformer_log_level=self.cfg.get('apex_transformer_log_level', 30),
        )
```

This should have the option to set expert_parallel_size here but it's currently not supported. Any ETA when changes to NeMo will add this?



Training will fail with the following error: 

[1,1] File "~/Megatron-LM/megatron/core/parallel_state.py", line 550, in get_tensor_and_expert_parallel_group
[1,1]<stdout>:    assert (
[1,1]<stdout>:AssertionError: tensor and expert parallel group is not initialized

which makes sense since the option for expert parallel is not supported in the initialize_model_parallel_for_nemo() function.

Setup for testing uses latest updates in both Megatron-LM and NeMo-Megatron

