Rewritten batch support in pipelines.
Batch support in Pipeline was confusing and not well tested. 

This PR rewrites all the content of `DefaultArgumentHandler` which handles most of the input conversions (args, kwargs, batched, etc.) and brings unit tests on this specific class.

Signed-off-by: Morgan Funtowicz <morgan@huggingface.co>
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4154?src=pr&el=h1) Report
> Merging [#4154](https://codecov.io/gh/huggingface/transformers/pull/4154?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/818463ee8eaf3a1cd5ddc2623789cbd7bb517d02&el=desc) will **increase** coverage by `0.03%`.
> The diff coverage is `96.42%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4154/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4154?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4154      +/-   ##
==========================================
+ Coverage   78.79%   78.83%   +0.03%     
==========================================
  Files         114      114              
  Lines       18711    18726      +15     
==========================================
+ Hits        14743    14762      +19     
+ Misses       3968     3964       -4     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4154?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/pipelines.py](https://codecov.io/gh/huggingface/transformers/pull/4154/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9waXBlbGluZXMucHk=) | `76.27% <96.42%> (+1.32%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4154/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `92.77% <0.00%> (+0.16%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4154?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4154?src=pr&el=footer). Last update [818463e...95a257c](https://codecov.io/gh/huggingface/transformers/pull/4154?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Ok so this should be ready to merge, right @mfuntowicz?<|||||>+1 Would love a merge here so I can more easily improve `TranslationPipeline`