ONNX Export for GPT models fails
**Describe the bug**

Using NeMo container 23.06 and the [nemo_gpt1.3B_fp16.nemo](https://huggingface.co/nvidia/nemo-megatron-gpt-1.3B/blob/main/) file located in the NVIDIA HuggingFace Hub, the following command fails to execute:

"python3 examples/nlp/language_modeling/megatron_export.py \
    model_type=gpt \
    onnx_model_file=nemo_gpt1.3B_fp16.onnx \
    gpt_model_file=nemo_gpt1.3B_fp16.nemo"

With the error:

`TypeError: Can't instantiate abstract class ModelPT with abstract methods list_available_models, setup_training_data, setup_validation_data`

It seems NeMo is failing on the line:

`ModelPT.restore_from("/path/to/.nemo")`


**Steps/Code to reproduce bug**

1. Pull NeMo container [23.06](https://catalog.ngc.nvidia.com/orgs/nvidia/containers/nemo/tags)
2. Pull the megatron-gpt1.3B model using `wget https://huggingface.co/nvidia/nemo-megatron-gpt-1.3B/resolve/main/nemo_gpt1.3B_fp16.nemo`
3. Execute:

python3 examples/nlp/language_modeling/megatron_export.py \
    model_type=gpt \
    onnx_model_file=nemo_gpt1.3B_fp16.onnx \
    gpt_model_file=nemo_gpt1.3B_fp16.nemo

**Expected behavior**

The script runs and produces a valid `.onnx` file at the desired location.

**Environment overview (please complete the following information)**

 - Fresh NeMo container 23.06

**Environment details**

**Additional context**