Cannot set max_position_embeddings to any desired value in T5Config
# 🐛 Bug

## Information

The model I am using (T5):

Language I am using the model on (English):

The problem arises when using:
* [ ] the official example scripts: (give details below)
* [x] my own modified scripts: (give details below)

```
config = T5Config(max_position_embeddings=1024)
```

## To reproduce

Steps to reproduce the behaviour:

1.Initiate T5Config
2. set max_position_embeddings any desired value like 1024, 2048 (as mentioned in docs)

This works perfectly fine with BertConfig

This is an error:
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```
Can't set max_position_embeddings with value 1024 for T5Config {
  "_num_labels": 2,
  "architectures": null,
  "bad_words_ids": null,
  "bos_token_id": null,
  "decoder_start_token_id": null,
  "do_sample": false,
  "early_stopping": false,
  "eos_token_id": 1,
  "finetuning_task": null,
  "id2label": {
    "0": "LABEL_0",
    "1": "LABEL_1"
  },
  "is_decoder": false,
  "is_encoder_decoder": true,
  "label2id": {
    "LABEL_0": 0,
    "LABEL_1": 1
  },
  "length_penalty": 1.0,
  "max_length": 20,
  "min_length": 0,
  "model_type": "t5",
  "no_repeat_ngram_size": 0,
  "num_beams": 1,
  "num_return_sequences": 1,
  "output_attentions": false,
  "output_hidden_states": false,
  "output_past": true,
  "pad_token_id": 0,
  "prefix": null,
  "pruned_heads": {},
  "repetition_penalty": 1.0,
  "task_specific_params": null,
  "temperature": 1.0,
  "top_k": 50,
  "top_p": 1.0,
  "torchscript": false,
  "use_bfloat16": false
}

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-5-44823d82a38e> in <module>()
----> 1 config = T5Config(max_position_embeddings=1024)

2 frames
/usr/local/lib/python3.6/dist-packages/transformers/configuration_utils.py in __init__(self, **kwargs)
    106         for key, value in kwargs.items():
    107             try:
--> 108                 setattr(self, key, value)
    109             except AttributeError as err:
    110                 logger.error("Can't set {} with value {} for {}".format(key, value, self))

AttributeError: can't set attribute
```
After further investigating in docs here is what I found (If I am wrong, then please correct me):
```
self.vocab_size = vocab_size
self.n_positions = n_positions
self.d_model = d_model
self.d_kv = d_kv
self.d_ff = d_ff
self.num_layers = num_layers
self.num_heads = num_heads
self.relative_attention_num_buckets = relative_attention_num_buckets
self.dropout_rate = dropout_rate
self.layer_norm_epsilon = layer_norm_epsilon
self.initializer_factor = initializer_factor

@property
def max_position_embeddings(self):
    return self.n_positions
```
Looking at code snippets I think we have to directly use n_positions instead of max_position_embeddings. I have tried this approach & **it worked**.

## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 2.8.0
- Platform: Linux-4.19.104+-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.6.9
- PyTorch version (GPU?): 1.5.0+cu101 (True)
- Tensorflow version (GPU?): 2.2.0-rc3 (True)
- Using GPU in script?: True

IMO the docstring is confusing here. This PR should fix it: #4422. 
& You are right you should change `max_position_embeddings` via `n_positions`.