Fix drop_path_rates argument passed to ConvNextStage
Author: alex-coniasse
Pull request: yes

# What does this PR do?

- Pass a sub-list of `drop_path_rates` to ConvNextStage  instead of a single element.
- Fix the following issue: 
Argument `drop_path_rates (List[float])` passed to `ConvNextStage` is a single float
when `drop_path_rate` is specified by the config.


Reproducer:
```
from transformers import ConvNextModel, ConvNextConfig

configuration = ConvNextConfig(drop_path_rate=0.1)
model = ConvNextModel(configuration)
```
Throws:
```
Traceback (most recent call last):
  File "repro-droprate.py", line 4, in <module>
    model = ConvNextModel(configuration)
  File "/home/alexandrec/.local/lib/python3.6/site-packages/transformers/models/convnext/modeling_convnext.py", line 311, in __init__
    self.encoder = ConvNextEncoder(config)
  File "/home/alexandrec/.local/lib/python3.6/site-packages/transformers/models/convnext/modeling_convnext.py", line 221, in __init__
    drop_path_rates=drop_path_rates[cur],
  File "/home/alexandrec/.local/lib/python3.6/site-packages/transformers/models/convnext/modeling_convnext.py", line 197, in __init__
    *[ConvNextLayer(config, dim=out_channels, drop_path=drop_path_rates[j]) for j in range(depth)]
  File "/home/alexandrec/.local/lib/python3.6/site-packages/transformers/models/convnext/modeling_convnext.py", line 197, in <listcomp>
    *[ConvNextLayer(config, dim=out_channels, drop_path=drop_path_rates[j]) for j in range(depth)]
TypeError: 'float' object is not subscriptable
```
Reproduced with Transformers 4.18.0 , Ubuntu 18.04 + Python 3.6.9

Fixes # (issue)


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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_16699). All of your documentation changes will be reflected on that endpoint.

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.

Thanks for pointing out, I'll remove the unclear `cur` variable and will fix it for the TF implementation as well.

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.

Closing as this was fixed in #17280