[BUG] GPT2BPETokenizer (and possibly others) missing decode and offsets methods

I've encountered an issue when running text generation with Megatron-LM. Apologies in advance if there are any mistakes — I'm a new user.

After successfully preprocessing the data using:

```bash
python tools/preprocess_data.py --tokenizer-type GPT2BPETokenizer
```

and completing pretraining, I tried running text generation using:

```bash
tools/run_text_generation_server.py
```

However, I received the following errors:

```
AttributeError: '_GPT2BPETokenizer' object has no attribute 'decode'
AttributeError: '_GPT2BPETokenizer' object has no attribute 'offsets'
```

These errors seem to originate from:

* [`tokenizer.py`, line 323](https://github.com/NVIDIA/Megatron-LM/blob/7f7ffcf895aba1e4f5739f27236031401ee32598/megatron/training/tokenizer/tokenizer.py#L323)
* [`run_text_generation_server_345M.sh`, line 25](https://github.com/NVIDIA/Megatron-LM/blob/7f7ffcf895aba1e4f5739f27236031401ee32598/examples/inference/run_text_generation_server_345M.sh#L25)

It seems like `_GPT2BPETokenizer` may be missing `decode` and `offsets` methods required by the inference script. Any guidance on how to resolve this would be greatly appreciated.

Start by reproducing the failure with tools/preprocess_data.py using GPT2BPETokenizer and tools/run_text_generation_server.py. Inspect megatron/training/tokenizer/tokenizer.py around line 323 and examples/inference/run_text_generation_server_345M.sh around line 25; done means text generation no longer raises missing decode or offsets errors.

Written by the indexing model from the issue text.