blenderbot checkpoint convert script has bug.
- error

![image](https://user-images.githubusercontent.com/38183241/122261506-70459700-cf0f-11eb-9c51-d30fc6362352.png)

- original parlai checkpoint

![image](https://user-images.githubusercontent.com/38183241/122261541-79ceff00-cf0f-11eb-938f-0be13cadd471.png)

- so, we should fix codes like below.
```python

def rename_layernorm_keys(sd):
    keys = [
        "encoder.norm_embeddings.weight",
        "encoder.norm_embeddings.bias",
        "decoder.norm_embeddings.weight",
        "decoder.norm_embeddings.bias",
    ]
    for k in keys:
        v = sd.pop(k)
        new_k = "model." + k.replace("norm_embeddings", "layer_norm")
        assert new_k not in sd
        sd[new_k] = v


IGNORE_KEYS = ["START"]
```
Now I have almost fixed the bug. I will PR soon.