Don't see how to use correct padding with QA pipeline
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## Details

In some cases, when using a **QuestionAnsweringPipeline**, I get an error similar to the following:

    *** ValueError: expected sequence of length 384 at dim 1 (got 260)

I've traced the problem to line 1496 of pipelines.py, and the change to [this commit](https://github.com/huggingface/transformers/commit/896300177bf9f35feac4698370212a80a5ab6138).  The troubled line is:

    fw_args = {k: torch.tensor(v, device=self.device) for (k, v) in fw_args.items()}

Basically, when trying to convert this **v** to a tensor an error is thrown because not every array in **v** has the same length - because the padding strategy has been changed (if I comment out the padding strategy in the call to **squad_convert_examples_to_features** then the default value `"max_length"` takes effect and there is no problem).  I guess this was done as some sort of optimization, but I'm not really sure how to use it.  Every other argument to **squad_convert_examples_to_features** will use a *kwarg*, but this one does not.  Maybe it should use a *kwarg* like everything else, so if you need the padding (or don't want to have to deal with it) you can set the **padding_strategy** as you like?  Or am I missing something?

### Minimal code to reproduce:

    from transformers import QuestionAnsweringPipeline, AutoModelForQuestionAnswering, AutoTokenizer
    model = AutoModelForQuestionAnswering.from_pretrained('twmkn9/distilbert-base-uncased-squad2')
    tokenizer = AutoTokenizer.from_pretrained('twmkn9/distilbert-base-uncased-squad2')
    
    # I've omitted the context for brevity.  You can, for example, take the **plot** section from [the matrix](https://en.wikipedia.org/wiki/The_Matrix)
    context = """..."""
    
    pipeline({"question":"what was Neo's job?", "context": context}) # error as described above

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