Weights of model checkpoint not initialized for RobertaModel for Bertscore
Hi @suzyahyah, I have created a Pull Request to filter out that warning message in this specific case, since the behavior is as expected and the warning message can only cause confusion for users (as in your case).
When applying bertscore out of the box, 

```Some weights of the model checkpoint at roberta-large were not used when initializing RobertaModel: ['lm_head.decoder.weight', 'lm_head.bias', 'lm_head.dense.bias', 'lm_head.layer_norm.bias', 'lm_head.dense.weight', 'lm_head.layer_norm.weight']```

Following the typical usage from https://huggingface.co/docs/datasets/loading_metrics.html

```
from datasets import load_metric
metric = load_metric('bertscore')

# Example of typical usage
for batch in dataset:
    inputs, references = batch
    predictions = model(inputs)
    metric.add_batch(predictions=predictions, references=references)
score = metric.compute(lang="en")
#score = metric.compute(model_type="roberta-large") # gives the same error
```

I am concerned about this because my usage shouldn't require any further fine-tuning and most people would expect to use BertScore out of the box? I realised the huggingface code is a wrapper around https://github.com/Tiiiger/bert_score, but I think this repo is anyway relying on the model code and weights from huggingface repo.... 

## Environment info
- `datasets` version: 1.7.0
- Platform: Linux-5.4.0-1041-aws-x86_64-with-glibc2.27
- Python version:  3.9.5
- PyArrow version: 3.0.0

Weights of model checkpoint not initialized for RobertaModel for Bertscore 
 When applying bertscore out of the box, 

```Some weights of the model checkpoint at roberta-large were not used when initializing RobertaModel: ['lm_head.decoder.weight', 'lm_head.bias', 'lm_head.dense.bias', 'lm_head.layer_norm.bias', 'lm_head.dense.weight', 'lm_head.layer_norm.weight']```

Following the typical usage from https://huggingface.co/docs/datasets/loading_metrics.html

```
from datasets import load_metric
metric = load_metric('bertscore')

# Example of typical usage
for batch in dataset:
    inputs, references = batch
    predictions = model(inputs)
    metric.add_batch(predictions=predictions, references=references)
score = metric.compute(lang="en")
#score = metric.compute(model_type="roberta-large") # gives the same error
```

I am concerned about this because my usage shouldn't require any further fine-tuning and most people would expect to use BertScore out of the box? I realised the huggingface code is a wrapper around https://github.com/Tiiiger/bert_score, but I think this repo is anyway relying on the model code and weights from huggingface repo.... 

## Environment info
- `datasets` version: 1.7.0
- Platform: Linux-5.4.0-1041-aws-x86_64-with-glibc2.27
- Python version:  3.9.5
- PyArrow version: 3.0.0
 
 Hi @suzyahyah, I have created a Pull Request to filter out that warning message in this specific case, since the behavior is as expected and the warning message can only cause confusion for users (as in your case).