Return attention_mask in FeatureExtractionPipeline output
### Feature request

Return `attention_mask` as one output of the FeatureExtractionPipeline so that padding token embeddings can be ignored.

### Motivation

**Who can help?**
@Narsil

When using the `FeatureExtractionPipeline` to generate sentence embeddings, the input to the pipeline processes a raw sentence with a tokenizer. The output of the pipeline is a tensor of shape `[1, seq_len, hidden_dim]`. If the input is padded, `seq_len` is equal to the `max_length` of the tokenizer or longest seq in the batch. 

However, when performing mean pooling of individual word embeddings to obtain the sentence embedding, one may want to use `attention_mask` in order to ignore the padding token embeddings (see the mean pooling example below). But, FeatureExtractionPipeline does not return `attention_mask` as part of its output.

```python
#Mean Pooling - Take attention mask into account for correct averaging
def mean_pooling(model_output, attention_mask):
    token_embeddings = model_output[0] #First element of model_output contains all token embeddings
    input_mask_expanded = attention_mask.unsqueeze(-1).expand(token_embeddings.size()).float()
    sum_embeddings = torch.sum(token_embeddings * input_mask_expanded, 1)
    sum_mask = torch.clamp(input_mask_expanded.sum(1), min=1e-9)
    return sum_embeddings / sum_mask
```


### Your contribution

I can submit a pull request to the issue if it sounds good to you!
This doesn't seem like a use-case for the pipeline though. Since you want access to the process inputs, you should just used the tokenizer and the model directly.<|||||>Your comment makes sense. As my goal aligns with the pipeline's main functionality, I think I will subclass `FeatureExtractionPipeline` and make small modifications to achieve my goal. Feel free to close the issue. Thank you!<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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