NMT adding pretrained HF encoders
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This PR will add HuggingFace Encoder support to NeMo NMT.

When training NeMo NMT models, users can now: 

- Download pretrained model from HF and use it for NMT automatically
- The NMT data preprocessor will automatically use the tokenizer associated with pretrained when preprocessing the data
- The HF encoder can be used with the pretrained weights or with randomly initialized weights
- The HF encoder can be configured from scratch.

# Implementation Details
Since our ultimate goal is to be able to use encoders from HuggingFace, Megatron-LM, and NeMo, we've added some new flags to NMT.

The `library` flag takes values: `huggingface`, `megatron` (when implemented), and `nemo`.
NMT has `library=nemo` by default to maintain backwards compatibility.

The `model_name` flag is used to indicate a _named_ model architecture. From HuggingFace, we have `bert_base_cased` for example.

We've also added the `pretrained` flag. This allows users to decide whether they want to download the named model's pretrained weights `pretrained=True` or simply instantiated the named model architecture with random weights `pretrained=False`.

If a user wants to use a model architecture from a specific library, then they can use `model_name=None` and add the custom configuration under the `encoder` configuration. For example:
```
encoder:
  _target_: transformers.BertConfig
  hidden_size: 1536
```
Also note that `shared_tokenizer` should `False` when using HuggingFace encoders since we do not support HuggingFace Decoders yet.

# Usage
We've provided a config file to use with HuggingFace encoders: examples/nlp/machine_translation/conf/huggingface.yaml

We can use that config file by specifying from the CLI: 
```
--config-path=conf \
--config-name=huggingface
```
## Pretrained HuggingFace Encoder
This will train an NMT model with a pretrained encoder model from HuggingFace and
a randomly initialized decoder from NeMo.
To train the encoder from scratch, set `encoder.pretrained=False`. 
```
MODEL_NAME = bert-base-cased
python examples/nlp/machine_translation/enc_dec_nmt.py \
	--config-path=conf \
	--config-name=huggingface \
        model.src_language=en \
        model.tgt_language=de \
	model.preproc_out_dir=/raid/data/hf_en_de_preproc_${MODEL_NAME} \
	model.shared_tokenizer=false \
	model.encoder_tokenizer.library=huggingface \
	model.encoder.library=huggingface \
	model.encoder.pretrained=true \
	model.encoder.model_name=${MODEL_NAME} \
	model.decoder_tokenizer.library=yttm \
	model.decoder_tokenizer.vocab_size=32000 \
	model.decoder.library=nemo \
	model.decoder.inner_size=4096 \
	model.decoder.num_attention_heads=16 \
	model.decoder.attn_layer_dropout=0.1 \
	model.decoder.ffn_dropout=0.1 \
	model.decoder.num_layers=6 \
        ...
```
## Custom HuggingFace Encoder
This will train an NMT model with a custom encoder model from HuggingFace and
a randomly initialized decoder from NeMo.
Note: we have to use `+` from the CLI if we are not adding the argument to the config file.
```
python examples/nlp/machine_translation/enc_dec_nmt.py \
	--config-path=conf \
	--config-name=huggingface \
        model.src_language=en \
        model.tgt_language=de \
	model.preproc_out_dir=/raid/data/hf_en_de_preproc_custom \
	model.shared_tokenizer=false \
	model.encoder_tokenizer.library=huggingface \
	model.encoder.library=huggingface \ 
        +model.encoder._target_=transformers.BertConfig \
        +model.encoder.hidden_size=1536 \
	model.decoder_tokenizer.library=yttm \
	model.decoder_tokenizer.vocab_size=32000 \
	model.decoder.library=nemo \
	model.decoder.inner_size=4096 \
	model.decoder.num_attention_heads=16 \
	model.decoder.attn_layer_dropout=0.1 \
	model.decoder.ffn_dropout=0.1 \
	model.decoder.num_layers=6 \
        ...
```