Support tp pp conversion
# What does this PR do ?

Adds support for changing pipeline parallel version post construction for GPT

**Collection**: [Core, NLP]

# Changelog 
- Add new script for pp conversion to avoid breaking old script (should probably be deprecated eventually when new script fully supports all functionality)
- Add some modifications to allow partially loading model parallel models into memory to extract parameters
- Add PP TP conversion support for both Megatron GPT and Megatron T5 (only when shared embeddings are used between encoder and decoder)
- Add support for forcing CPU construction of T5 and GPT models
- Add support for loading model based on AppState parameters instead of Trainer parameters.

# Usage

```python
Usage:

# Megatron GPT
python megatron_change_num_partitions.py \
    --model_file=PATH_TO_SRC_FILE \
    --target_file=PATH_TO_TGT_FILE \
    --tensor_model_parallel_size=1 \
    --target_tensor_model_parallel_size=1 \
    --pipeline_model_parallel_size=1 \
    --target_pipeline_model_parallel_size=1 \
    --precision=bf16

# Megatron T5
python megatron_change_num_partitions.py \
    --model_file=PATH_TO_SRC_FILE \
    --target_file=PATH_TO_TGT_FILE \
    --model_class="nemo.collections.nlp.models.language_modeling.megatron_t5_model.MegatronT5Model" \
    --tensor_model_parallel_size=1 \
    --target_tensor_model_parallel_size=1 \
    --pipeline_model_parallel_size=1 \
    --target_pipeline_model_parallel_size=1 \
    --target_pipeline_model_parallel_split_rank=0 \
    --precision=bf16

# NOTE: When converting large models, always ensure that you pre-extract the nemo model and then only perform conversion

$ mkdir "unpacked_nemo_file"
$ tar -xvf "<path to nemo file>" -C "<absolute path to pwd>/unpacked_nemo_file/"

python megatron_change_num_partitions.py \
    ...
    --model_extracted_dir="<Absolute path to pwd>/unpacked_nemo_file/"

# NOTE: Conversion of other model types. 
# Default model type is MegatronGPTModel, if you want another model you need to pass classpath of the model
# For example - MegatronT5Model - 

python megatron_change_num_partitions.py \
    ...
    --model_class="nemo.collections.nlp.models.language_modeling.megatron_t5_model.MegatronT5Model"

# Additional arguments:

--num_gpu_per_node: Number of GPUs per node. Default is 8.
--megatron_legacy: Whether the model is a legacy Megatron model or not. Default is False. May be unsuported for 
    Pipeline Parallelism change.
--tokenizer_model_path: Path to tokenizer model. Default is None. When not None, overrides the tokenizer model path
    in the model config.
--tokenizer_vocab_file: Path to tokenizer vocab file. Default is None. When not None, overrides the tokenizer vocab
    file in the model config.

```

# Before your PR is "Ready for review"
**Pre checks**:
- [ ] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [ ] Did you write any new necessary tests?
- [ ] Did you add or update any necessary documentation?
- [ ] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
  - [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [x] New Feature
- [ ] Bugfix
- [ ] Documentation

