Use _get_mlp_module_spec from Megatron Core rather than redefine locally
# What does this PR do ?

Reuse `_get_mlp_module_spec ` from Megatron Core to improve stability. This also fixes Mixtral loading bug as the current implementation fails with:
```
  File "/opt/megatron-lm/megatron/core/transformer/moe/moe_layer.py", line 109, in __init__
    self.experts = build_module(self.submodules.experts, self.num_local_experts, self.config)
  File "/opt/megatron-lm/megatron/core/transformer/spec_utils.py", line 82, in build_module
    module = import_module(spec_or_module.module)
AttributeError: 'MLPSubmodules' object has no attribute 'module' when instantiating MoELayer when instantiating TransformerLayer
```
due to recent refactoring in Megatron https://github.com/NVIDIA/Megatron-LM/commit/aa2a45dd44516925ba5c0579eb262caf48a81a1b.

**Collection**: NLP / LLM

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