Fix encoder->decoder typo bug in convert_t5x_checkpoint_to_pytorch.py
Author: soyoung97
Pull request: yes

# What does this PR do?
The convert_t5x_checkpoint_to_pytorch is used to convert t5x models into pytorch models.
However, it contains a typo at [line 142](https://github.com/huggingface/transformers/blob/main/src/transformers/models/t5/convert_t5x_checkpoint_to_pytorch.py#L142): The wi weights in **decoder** is converted to weights in **encoder**, and it makes the following errors when we run the script **for t5 v1.0 models** (where Split MLP layers is false and uses T5DenseActDense(wi) instead of T5DenseGatedActDense(wi_0, wi_1) is run:

```
 File "/opt/conda/envs/myenv/lib/python3.10/site-packages/torch/nn/modules/module.py", line 1497, in load_state_dict
    raise RuntimeError('Error(s) in loading state_dict for {}:\n\t{}'.format(
RuntimeError: Error(s) in loading state_dict for T5ForConditionalGeneration:
        Missing key(s) in state_dict: "decoder.block.0.layer.2.DenseReluDense.wi.weight", "decoder.block.1.layer.2.DenseReluDense.wi.weight", "decoder.block.2.layer.2.DenseReluDense.wi.weight", "decoder.block.3.layer.2.DenseReluDense.wi.weight", "decoder.block.4.layer.2.DenseReluDense.wi.weight", "decoder.block.5.layer.2.DenseReluDense.wi.weight", "decoder.block.6.layer.2.DenseReluDense.wi.weight", "decoder.block.7.layer.2.DenseReluDense.wi.weight", "decoder.block.8.layer.2.DenseReluDense.wi.weight", "decoder.block.9.layer.2.DenseReluDense.wi.weight", "decoder.block.10.layer.2.DenseReluDense.wi.weight", "decoder.block.11.layer.2.DenseReluDense.wi.weight".
        Unexpected key(s) in state_dict: "encoder.block.0.layer.2.DenseReluDense.wi.weight", "encoder.block.1.layer.2.DenseReluDense.wi.weight", "encoder.block.2.layer.2.DenseReluDense.wi.weight", "encoder.block.3.layer.2.DenseReluDense.wi.weight", "encoder.block.4.layer.2.DenseReluDense.wi.weight", "encoder.block.5.layer.2.DenseReluDense.wi.weight", "encoder.block.6.layer.2.DenseReluDense.wi.weight", "encoder.block.7.layer.2.DenseReluDense.wi.weight", "encoder.block.8.layer.2.DenseReluDense.wi.weight", "encoder.block.9.layer.2.DenseReluDense.wi.weight", "encoder.block.10.layer.2.DenseReluDense.wi.weight", "encoder.block.11.layer.2.DenseReluDense.wi.weight".
```

The following is the changed part:
```
            if split_mlp_wi:

                new[f"decoder.block.{i}.layer.2.DenseReluDense.wi_0.weight"] = wi[0].T

                new[f"decoder.block.{i}.layer.2.DenseReluDense.wi_1.weight"] = wi[1].T

            else:

                new[f"encoder.block.{i}.layer.2.DenseReluDense.wi.weight"] = wi.T
```

When changing the following line from 
```
new[f"encoder.block.{i}.layer.2.DenseReluDense.wi.weight"] = wi.T
```
to 
```
new[f"decoder.block.{i}.layer.2.DenseReluDense.wi.weight"] = wi.T
``` 
the code works fine.


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Related pull request seems to be [this one](https://github.com/huggingface/transformers/pull/20801), 
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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_26587). All of your documentation changes will be reflected on that endpoint.

I think it went unnoticed because v1.0 model conversion is not used frequently than v1.1 models. Thanks a lot for the fast review!!