Introduce a new tensor type for return_tensors on tokenizer for NumPy
Author: mfuntowicz
Pull request: yes

Two changes in this PR:

- As we're introducing more than two tensor backend alternatives I created an enum `TensorType` listing all the possible tensor we can create `TensorType.TENSORFLOW`, `TensorType.PYTORCH`, `TensorType.NUMPY`. This might help newcomers who don't know about `"tf"`, `"pt"`.

   _->Note: TensorType are compatible with previous  `"tf"`, `"pt"` and now `"np"` str to allow backward compatbility (+unittest)_

- Numpy is now a possible target when creating tensors. This is usefull for JAX :) 

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4585?src=pr&el=h1) Report
> Merging [#4585](https://codecov.io/gh/huggingface/transformers/pull/4585?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/3e3e552125e86824239e445dd3c659df0aea4db9&el=desc) will **decrease** coverage by `0.00%`.
> The diff coverage is `94.11%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4585/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4585?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4585      +/-   ##
==========================================
- Coverage   78.09%   78.09%   -0.01%     
==========================================
  Files         123      123              
  Lines       20624    20622       -2     
==========================================
- Hits        16106    16104       -2     
  Misses       4518     4518              
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4585?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4585/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `89.84% <93.93%> (+0.32%)` | :arrow_up: |
| [src/transformers/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/4585/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9fX2luaXRfXy5weQ==) | `99.13% <100.00%> (ø)` | |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4585/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.01% <0.00%> (-0.66%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4585/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.53% <0.00%> (+0.11%)` | :arrow_up: |

------

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@LysandreJik I pushed a initial version of `test_np_encode_plus_sent_to_model` which converts input to numpy tensor. 

For the moment we don't have any model to forward through (JAX/Flax PR is not merged). I added a note to complete the unittests when we have the full pipeline available.