Update model configs - Allow setters for common properties 
# Update model configs - Allow setters for common properties 
Not all models use the same naming for config values, e.g. `hidden_size` is called `n_embd` in GPT2Config. So far, getters had been implemented in the config classes to allow that a GPT2Config can be accessed via `config.hidden_size`.

But the setters were missing, so that this code fails so far:
```python
from transformers import GPT2Config
config = GPT2Config()
config.hidden_size = 4 # Fails

config = GPT2Config(hidden_size =4) # Fails
```


## Changes
This PR adds an `attribute_map` to the config classes that maps the config parameters. For GPT2, this map looks like this:
```python
attribute_map = {"hidden_size": "n_embd",
                     "max_position_embeddings": "n_positions",
                     "num_attention_heads": "n_head",
                     "num_hidden_layers": "n_layer"
                     }
```

The `PretrainedConfig` class overwrites the get & set attribute to check for the mappings in the attribute_map:
```python
    def __setattr__(self, key, value):
        if key in super().__getattribute__('attribute_map'):
            key = super().__getattribute__('attribute_map')[key]
        super().__setattr__(key, value)

    def __getattribute__(self, key):
        if key != 'attribute_map' and key in super().__getattribute__('attribute_map'):
            key = super().__getattribute__('attribute_map')[key]
        return super().__getattribute__(key)
```

## Advantages
- Setters work, i.e. you can use `config.hidden_size = 4` and `GPT2Config(hidden_size=4)`
- No need to write individual getter- or setter-methods in the config classes. They are derived from the `attribute_map`


## Detailed changes
-  `PretrainedConfig`: Add `__setattr__` and `__getattribute__` methods. Added docstring for `attribute_map`
-  `GPT2Config`: Add attribute map, remove old getters
- `test_configuration_common.py`: Update `create_and_test_config_common_properties` method so that it tests that setters exist and work

## ~~Work in Progress~~

~~So far I only updated the GPT2Config to get your feedback. Unit-Tests for other config classes that have not yet been updated (i.e. don't provide setters for the common fields) like the GPTNeo config class will fail.~~

~~Once the design of the solution is approved, I will update all other config classes.~~

Update: All config classes updated

## Fixes
- #12907
- #12183

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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      to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?


## Who can review?

@sgugger @LysandreJik @NielsRogge 

## Code to test the change
Besides  the unit tests, you can use this code to test the changes quickly:
```python
from transformers import GPT2Config

config = GPT2Config()

config.hidden_size = 4
print("Hidden size", config.hidden_size, config.n_embd)

config.n_positions = 65
print("n_positions", config.max_position_embeddings, config.n_positions)

config.max_position_embeddings = 123
print("n_positions", config.max_position_embeddings, config.n_positions)


print("\n\n================\n\n")

## Note: conflicting arguments: hidden_size and n_embd are identical fields
# In that case, the synonym (hidden_size) will have higher priority
config = GPT2Config(hidden_size=4, n_embd=20, max_position_embeddings=80)
print("Hidden size", config.hidden_size, config.n_embd)
print("n_positions", config.max_position_embeddings, config.n_positions)

print("Export to json")
config.save_pretrained(".")

## Load config
print("Load from disc")
config = GPT2Config.from_pretrained('.')
print("Hidden size", config.hidden_size, config.n_embd)
print("n_positions", config.max_position_embeddings, config.n_positions)
assert config.hidden_size == config.n_embd
assert config.hidden_size == 4

assert config.max_position_embeddings == config.n_positions
assert config.max_position_embeddings == 80
```
> The design looks good to me! I think we could have a few more common attributes, since we are in the process of adding them:
> 
> * the vocab size (seems to be pretty consistent)
> * the embedding size
> * the inner size for the feed-forward layers
> 
> Those on top of `max_position_embeddings` should all be included in `common_properties` so that we are sure they are common to each model.

My idea was to put this into an independent, new PR and too keep this PR focused on just changing the getter / setters.

My plan is to come up with some scheme which attributes should be common. Here we can differentiate between model types: text (differentiated between encoder only and encoder-decoder), image, audio.

I analyzed all 50+ config classes and these are the most common fields:
```
model_type 55
vocab_size 51
architectures 49
pad_token_id 42
max_position_embeddings 41
num_hidden_layers 40
initializer_range 36
eos_token_id 34
bos_token_id 32
hidden_size 32
layer_norm_eps 32
hidden_act 30
intermediate_size 30
num_attention_heads 29
hidden_dropout_prob 28
attention_probs_dropout_prob 26
transformers_version 25
type_vocab_size 23
attention_dropout 22
gradient_checkpointing 21
dropout 19
activation_dropout 18
d_model 17
init_std 17
activation_function 16
```

But as mentioned, I would put this in another PR.<|||||>Hi @sgugger @LysandreJik @patil-suraj @patrickvonplaten 

I also updated all other config classes so that they all use the `attribute_map` so that common properties (like `hidden_size`) can also be set (`config.hidden_size = 123`) or passed as argument (`MyConfigClass(hidden_size = 123)`).

I kept the behavior for the config classes as is, i.e. no new getter-methods were added, config classes were just extended to allow setting of the common properties.

If a setter method cannot be implemented for a class, an exception is raised:
https://github.com/huggingface/transformers/blob/c8973d1b5b2a498703e4308cba5056b5cbdaef12/src/transformers/models/funnel/configuration_funnel.py#L176


 
All unit tests are passing.

Would be happy if you could have a look at this PR.<|||||>@sgugger Will add a note to the docs

@patrickvonplaten Throwing an error is not easy.

`GPT2Config` defines `n_embd=768` in the `__init__` method, so:
`config = GPT2Config(hidden_size=4)`
and
`config = GPT2Config(hidden_size=4, n_embd=768)`

are identical calls of the method. We would expect method 1 to work.

In order to throw an exception for method 2, we could do:
- Replace all default parameters with None, see if `hidden_size` is not set, then set `n_embd` to 768 => Major refactoring on all config classes would be needed with quite a lot of overhead. Further, default parameters would no longer be visible from the definition of the method.
- Check if `n_embd != hidden_size  and n_embd != 768` => `config = GPT2Config(hidden_size=4, n_embd=8)` would throw an error, but `config = GPT2Config(hidden_size=4, n_embd=768)` would not raise an error (also not a nice solution). Also major refactoring would be needed as we would need to keep track of the default values for all parameters.

Do you have other ideas how this could be checked?<|||||>@sgugger 
I updated the docs:
transformers/docs/source/main_classes/configuration.rst

And added a section on the common attributes. Please have a look.<|||||>Hi,
I just updated the PR with the newest commits from the master branch.

However, now the run_examples_torch fails in CircleCI:
```
==================================== ERRORS ====================================
______________ ERROR collecting examples/pytorch/test_examples.py ______________
ImportError while importing test module '/home/circleci/transformers/examples/pytorch/test_examples.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/local/lib/python3.6/importlib/__init__.py:126: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
examples/pytorch/test_examples.py:51: in <module>
    import run_image_classification
examples/pytorch/image-classification/run_image_classification.py:27: in <module>
    from torchvision.transforms import (
E   ModuleNotFoundError: No module named 'torchvision'
```

Not sure why this happens, as this PR is not touching run_image_classification.py

Is this an issue with CircleCI or with the specific unit test?<|||||>Hi @nreimers, it's not related to this PR. That test fails because `torchvision` is not installed on the CI ( which is required in `run_image_classification.py`) for examples test. I've proposed a fix here #13438<|||||>Hi @patil-suraj 
Thanks for the quick response.

What are the next steps for this PR? Wait until #13438 is merged and then, when all tests are passing, merging this PR?

Who will be merging this PR? Should I do it once all tests are passing?<|||||>The failed test is not related to this PR and all of us has approved this PR, so feel free to merge if everything is ready :) 