Correct way of loading pretrained model led-base-16384

This [issue](https://github.com/allenai/longformer/issues/192) discusses about the difference between HuggingFace LED and AllenAI LED. What is the correct way of loading AllenAI's pretrained model `led-base-16384`?

`Approach 1 using HuggingFace LED:`
Using `transformers v4.9.1`
```
from transformers import AutoTokenizer, AutoModelForSeq2SeqLM

tokenizer = AutoTokenizer.from_pretrained("allenai/led-base-16384")

model = AutoModelForSeq2SeqLM.from_pretrained("allenai/led-base-16384", gradient_checkpointing=True)
```

`Approach 2 using AllenAI LED:`
Using transformer version suggested in `requirements.txt`:
`git+http://github.com/ibeltagy/transformers.git@longformer_encoder_decoder#egg=transformers`

```
from transformers import AutoTokenizer
from longformer.longformer_encoder_decoder import LongformerEncoderDecoderForConditionalGeneration
tokenizer = AutoTokenizer.from_pretrained("allenai/led-base-16384")
model = LongformerEncoderDecoderForConditionalGeneration.from_pretrained("allenai/led-base-16384", gradient_checkpointing=True)
```

`Results:`
Approach 1 seems to work but I am not sure whether it is correct because we are loading AllenAI's pretrained LED using HuggingLED which has different attention window size.

Approach 2 produces the error:
```
File "/Users/krishna/opt/anaconda3/envs/CiteKP/lib/python3.8/site-packages/transformers/configuration_utils.py", line 353, in get_config_dict
raise EnvironmentError
OSError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "LED_download.py", line 11, in
tokenizer = AutoTokenizer.from_pretrained("allenai/led-base-16384")
File "/Users/krishna/opt/anaconda3/envs/CiteKP/lib/python3.8/site-packages/transformers/tokenization_auto.py", line 209, in from_pretrained
config = AutoConfig.from_pretrained(pretrained_model_name_or_path, **kwargs)
File "/Users/krishna/opt/anaconda3/envs/CiteKP/lib/python3.8/site-packages/transformers/configuration_auto.py", line 272, in from_pretrained
config_dict, _ = PretrainedConfig.get_config_dict(pretrained_model_name_or_path, **kwargs)
File "/Users/krishna/opt/anaconda3/envs/CiteKP/lib/python3.8/site-packages/transformers/configuration_utils.py", line 362, in get_config_dict
raise EnvironmentError(msg)
OSError: Can't load config for 'allenai/led-base-16384'. Make sure that:

- 'allenai/led-base-16384' is a correct model identifier listed on 'https://huggingface.co/models'

- or 'allenai/led-base-16384' is the correct path to a directory containing a config.json file
```
I think the error is because the latest pretrained model `allenai/led-base-16384` is not compatible with the transformers version (i.e. `v3.1.0`) mentioned in the `requirements.txt`?