Support Changing Number of Tensor Parallel Partitions for Megatron
Added `megatron_change_num_partitions.py` that supports converting a Megatron model with an arbitrary number of Tensor Parallel (TP) partitions N to any other number of TP partitions M.

In order to do so, the script intermediately converts all models to TP=1.
All the models/tensors are created/loaded on CPU.

Usage:
```
python examples/nlp/language_modeling/megatron_change_num_partitions.py \
   --model_file=PATH_TO_SRC_FILE \
   --target_file=PATH_TO_TGT_FILE \
   --tensor_model_parallel_size=2 \
   --target_tensor_model_parallel_size=1
```
