Support single token decode for `CodeGenTokenizer`
Author: cmathw
Pull request: yes

# What does this PR do?

This PR should fix #28627 by first converting token_ids to a list in the the `decode` method of `CodeGenTokenizer` class. No new tests added but happy to write some if need be.

## Example

```python3
from transformers.models.auto.tokenization_auto import AutoTokenizer

phi_tokenizer = AutoTokenizer.from_pretrained("microsoft/phi-2", 
                                              add_bos_token=True, 
                                              use_fast=False, 
                                              trust_remote_code=True)

gpt2_tokenizer = AutoTokenizer.from_pretrained("gpt2", 
                                               add_bos_token=True, 
                                               use_fast=False, 
                                               trust_remote_code=True)

a = "The cat sat on the mat"
gpt2_tokens = gpt2_tokenizer(a, return_tensors="pt")["input_ids"][0] # torch.Size([7])
gpt2_str_tokens = gpt2_tokenizer.batch_decode(gpt2_tokens) # Essentially: [gpt2_tokenizer.decode(seq) for seq in gpt2_tokens]
print(gpt2_str_tokens) # <-- This is fine and will output: ['
', 'The', ' cat', ' sat', ' on', ' the', ' mat']

gpt2_single_decode = [gpt2_tokenizer.decode(gpt2_tokens[0])]
print(gpt2_single_decode) # <-- Decoding a 0-D tensor, this is fine and will output: ['
']

phi_tokens = phi_tokenizer(a, return_tensors="pt")["input_ids"][0] # torch.Size([7])
phi_str_tokens = phi_tokenizer.batch_decode(phi_tokens) # Essentially: [phi_tokenizer.decode(seq) for seq in phi_tokens]
print(phi_str_tokens) # <-- Cannot do this due to below...

phi_single_decode = [phi_tokenizer.decode(phi_tokens[0])]
print(phi_single_decode) # <-- Cannot decode a 0-D Tensor, hence cannot do above either
single_tok = phi_tokens[0].detach().cpu().tolist()

gpt2_single_decode = [gpt2_tokenizer.decode(gpt2_tokens)]
phi_single_decode = [phi_tokenizer.decode(phi_tokens)]
```

## Output before fix:

```bash
TypeError: iteration over a 0-d tensor
```

## Output after fix:
```bash
['
', 'The', ' cat', ' sat', ' on', ' the', ' mat']
['
']
['
', 'The', ' cat', ' sat', ' on', ' the', ' mat']
['
']
```


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

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