Megatron BART
# What does this PR do ?

This PR adds support in BART Megatron model <https://arxiv.org/abs/1910.13461>.

**Collection**: `nemo.collections.nlp.models.language_modeling.megatron_bart_model`

# Usage

```
python /code/examples/nlp/language_modeling/megatron_bart_pretraining.py \
	--config-path=/code/examples/nlp/language_modeling/conf \
	--config-name=megatron_bart_config \
	trainer.num_nodes=1 \
	trainer.devices=1 \
	trainer.max_epochs=null \
	trainer.max_steps=${MAX_STEPS} \
	trainer.val_check_interval=${VAL_CHECK_INTERVAL} \
	trainer.accumulate_grad_batches=${ACCUMULATE_GRAD_BATCHES} \
	trainer.precision=16 \
	model.micro_batch_size=${MICRO_BATCH_SIZE} \
	model.tensor_model_parallel_size=${TENSOR_MODEL_PARALLEL_SIZE} \
	model.max_position_embeddings=${SEQ_LENGTH} \
	model.seq_length=${SEQ_LENGTH} \
	model.hidden_size=${HIDDEN_SIZE} \
	model.num_layers=${NUM_LAYERS} \
	model.num_attention_heads=${NUM_ATTENTION_HEADS} \
	model.init_method_std=0.015 \
	model.data.data_prefix=[1.0,/preproc_data/train/my-t5_test_bert_tokenizer_text_document] \
	model.data.seq_length=${SEQ_LENGTH} \
```