Reorganize repo
# What does this PR do?

This PR reorgnaizes the structure of the repository by putting all model-related files (modeling, configuration, tokenization, conversion) in subfolders.

**Breaking change**

This breaks any import for model/config/tokenizer objects that is not done at the top level:
```
from transformers import BertModel
```
works but not
```
from transformers.modeling_bert import BertModel
```
It needs to be updated to
```
from transformers.models.bert.modeling_bert import BertModel
```

Internally, after this PR is merged the following will need fixing:
- the check_repo script does not properly finds the models now (so it does not check if they are properly tested/documented/ in auto classes)
- the new model template needs to be updated

The other internal scripts work as usual.
Got approval offline from @LysandreJik so merging. If anything needs fixing, we'll do so tomorrow morning!<|||||>The GPU tests throw this error: 

```
2020-11-17 11:24:06.538757: I tensorflow/stream_executor/platform/default/dso_loader.cc:48] Successfully opened dynamic library libcudart.so.10.1
Traceback (most recent call last):
  File "/home/hf/actions-runner_transformers/_work/transformers/transformers/.env/bin/transformers-cli", line 5, in <module>
    from transformers.commands.transformers_cli import main
  File "/home/hf/actions-runner_transformers/_work/transformers/transformers/.env/lib/python3.7/site-packages/transformers/__init__.py", line 34, in <module>
    from .data import (
  File "/home/hf/actions-runner_transformers/_work/transformers/transformers/.env/lib/python3.7/site-packages/transformers/data/__init__.py", line 6, in <module>
    from .processors import (
  File "/home/hf/actions-runner_transformers/_work/transformers/transformers/.env/lib/python3.7/site-packages/transformers/data/processors/__init__.py", line 6, in <module>
    from .squad import SquadExample, SquadFeatures, SquadV1Processor, SquadV2Processor, squad_convert_examples_to_features
  File "/home/hf/actions-runner_transformers/_work/transformers/transformers/.env/lib/python3.7/site-packages/transformers/data/processors/squad.py", line 10, in <module>
    from ...models.bert.tokenization_bert import whitespace_tokenize
ModuleNotFoundError: No module named 'transformers.models'
```

https://github.com/huggingface/transformers/runs/1411825179

Not 100% sure what's going on there<|||||>Normal import throw this error aswell:

```
Successfully installed sacremoses-0.0.43 tokenizers-0.9.4 transformers-4.0.0.dev0
---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
<ipython-input-2-11bca228d6cd> in <module>()
      1 get_ipython().system(' pip install ./transformers/')
----> 2 import transformers

3 frames
/usr/local/lib/python3.6/dist-packages/transformers/__init__.py in <module>()
     32 
     33 # Data
---> 34 from .data import (
     35     DataProcessor,
     36     InputExample,

/usr/local/lib/python3.6/dist-packages/transformers/data/__init__.py in <module>()
      4 
      5 from .metrics import glue_compute_metrics, xnli_compute_metrics
----> 6 from .processors import (
      7     DataProcessor,
      8     InputExample,

/usr/local/lib/python3.6/dist-packages/transformers/data/processors/__init__.py in <module>()
      4 
      5 from .glue import glue_convert_examples_to_features, glue_output_modes, glue_processors, glue_tasks_num_labels
----> 6 from .squad import SquadExample, SquadFeatures, SquadV1Processor, SquadV2Processor, squad_convert_examples_to_features
      7 from .utils import DataProcessor, InputExample, InputFeatures, SingleSentenceClassificationProcessor
      8 from .xnli import xnli_output_modes, xnli_processors, xnli_tasks_num_labels

/usr/local/lib/python3.6/dist-packages/transformers/data/processors/squad.py in <module>()
      8 
      9 from ...file_utils import is_tf_available, is_torch_available
---> 10 from ...models.bert.tokenization_bert import whitespace_tokenize
     11 from ...tokenization_utils_base import BatchEncoding, PreTrainedTokenizerBase, TruncationStrategy
     12 from ...utils import logging

ModuleNotFoundError: No module named 'transformers.models'
```
<|||||>I think maybe the `models` folder needs a `__init__.py`<|||||>I'm always confused by why Python lets it works in some cases and sometimes not. Will push an init directly on master.