Populate the GPT/BERT `list_available_models` with uploaded models
# What does this PR do?


Populate the GPT/BERT `list_available_models` with uploaded models. So, user can see all the models available by
```python
MegatronBertModel.list_available_models()
```
User can easily load the model by
```python
MegatronBertModel.from_pretrained('megatron_bert_345m_cased', trainer)
```

