Fix the behaviour of DefaultArgumentHandler (removing it).
Author: Narsil
Pull request: yes

# What does this PR do?

This PR will attempt to fix some clearly wrong error message in what I think should be
valid calls.

The decision to remove `DefaultArgumentHandler` comes from the fact that the real usage actually lied for only QuestionAnswering and ZeroShot which already have their own handler.

Having real python handle arguments and errors seems way more predictable and remove `*args` from function
signatures makes code more readable I think. We need to be very careful though as arguments number need to be in sync
otherwise errors can happen (This is due to mix of positional arguments, named positional arguments and generic keyword arguments being used together).

For the reader, the call order of functions is something like

```python
SpecificPipeline.__call__(myargument1, myargument2, **kwargs)
# Which calls
Pipeline.__call__(*args, **kwargs)
# Which in turn calls
SpecificPipeline._parse_and_tokenize(my_argument1, my_argument2, **kwargs)
```

Smaller changes for QoL where I tried to normalize inputs as early on as possible in the call stack (i.e SpecificPipeline.__call__) so we don't have to do it over and over.


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Comments:
Sure before the change

```python
from transformers import pipeline

pipe = pipeline(task='fill-mask', model='bert-base-uncased')
pipe("I am a real [MASK]", targets=["superhero", "legend"])
# [{'sequence': '[CLS] i am a real superhero [SEP]',
#  'score': 1.21390044682812e-07,
#  'token': 16251,
#  'token_str': 'superhero'},
# {'sequence': '[CLS] i am a real legend [SEP]',
#  'score': 4.292454747201191e-08,
#  'token': 5722,
#  'token_str': 'legend'}]

pipe("I am a real [MASK]", otherarg=True)
ValueError                                Traceback (most recent call last)
<ipython-input-13-4784fa412984> in <module>
----> 1 pipe("I am a real [MASK]", otherarg=True)

~/.pyenv/versions/3.8.5/lib/python3.8/site-packages/transformers/pipelines.py in __call__(self, targets, *args, **kwargs)
   1201             - **token** (:obj:`str`) -- The predicted token (to replace the masked one).
   1202         """
-> 1203         inputs = self._parse_and_tokenize(*args, **kwargs)
   1204         outputs = self._forward(inputs, return_tensors=True)
   1205 

~/.pyenv/versions/3.8.5/lib/python3.8/site-packages/transformers/pipelines.py in _parse_and_tokenize(self, padding, add_special_tokens, *args, **kwargs)
    625         """
    626         # Parse arguments
--> 627         inputs = self._args_parser(*args, **kwargs)
    628         inputs = self.tokenizer(
    629             inputs,

~/.pyenv/versions/3.8.5/lib/python3.8/site-packages/transformers/pipelines.py in __call__(self, *args, **kwargs)
    179     def __call__(self, *args, **kwargs):
    180         if len(kwargs) > 0 and len(args) > 0:
--> 181             raise ValueError("Pipeline cannot handle mixed args and kwargs")
    182 
    183         if len(kwargs) > 0:

ValueError: Pipeline cannot handle mixed args and kwargs

```

And afterwards:

```python
from transformers import pipeline

pipe = pipeline(task='fill-mask', model='bert-base-uncased')
pipe("I am a real [MASK]", otherarg=True)
# [{'sequence': '[CLS] i am a real. [SEP]',
#  'score': 0.94329434633255,
#  'token': 1012,
#  'token_str': '.'},
# {'sequence': '[CLS] i am a real ; [SEP]',
#  'score': 0.02879592962563038,
#  'token': 1025,
#  'token_str': ';'},
# {'sequence': '[CLS] i am a real! [SEP]',
#  'score': 0.022438935935497284,
 # 'token': 999,
#  'token_str': '!'},
# {'sequence': '[CLS] i am a real? [SEP]',
#  'score': 0.00518036400899291,
#  'token': 1029,
#  'token_str': '?'},
# {'sequence': '[CLS] i am a real... [SEP]',
#  'score': 3.598905823309906e-05,
#  'token': 2133,
#  'token_str': '...'}]
```

Should I merge ?

I could start that.

(no need to do it in this PR, it can wait :)