GPT2 - Remove else branch adding 0 to the hidden state if token_type_embeds is None.
Currently, when `token_type_embeds` is `None` we set its value to `0` and add it to the triplet `inputs_embeds + position_embeds + token_type_embeds`.

This can be simplified to: 
- avoid summing 0 over many elements
- avoid using raw Python scalar value which cannot be traced by TorchScript / ONNX when exporting.

Leading to: 

> [ONNXRuntimeError] : 1 : FAIL : TensorRT input: 200 has no shape specified. Please run shape inference on the onnx model first.

Signed-off-by: Morgan Funtowicz <funtowiczmo@gmail.com>