Change constant torch.tensor to torch.full
# What does this PR do?

Change `torch.tensor` to `torch.full` from GPT-2 to avoid CPU-GPU synchronization.

## Benchmarks with PyTorch Profiler

![GPT-2 with torch.tensor](https://user-images.githubusercontent.com/6321657/199923123-68838be1-e688-42ae-8b24-ce3d24f4589c.png)

Here's a trace of a single GPT-2 training iteration with 12 GPT-2 blocks, 2 GPUs, and DDP. 

From `_attn` function, there are two `torch.tensor` calls. Those invoke CPU to GPU memory movement, thus calling `cudaStreamSynchronize`.  

## How to fix 

From [PyTorch Recipes](https://pytorch.org/tutorials/recipes/recipes/tuning_guide.html#avoid-unnecessary-cpu-gpu-synchronization), we can avoid CPU-GPU synchronization by directly calling `torch.full` instead of `torch.tensor` or `torch.to`. Since two `torch.tensor` create constant tensors, we can change those into `torch.full([], ...)`, and it will behave the same way.

![GPT-2 without torch.tensor](https://user-images.githubusercontent.com/6321657/199923286-a4f4d423-b9ba-496b-8f7d-7f2cf731de02.png)

After the patch, every `cudaStreamSynchronize` is gone, and the duration of a single iteration is reduced by 0.5%.

Fixes # (issue)


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      Pull Request section?
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      [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.
@LysandreJik 


_The documentation is not available anymore as the PR was closed or merged._<|||||>For FX, I think this is already tested in the CI so I guess it does not break things. 
For the ONNX export, it's not tested but it should not break things IMO.<|||||>Following the change, the training with ONNX Runtime breaks as `mask_value` and `attn_weights` don't have the same dtype after being traced. Will open a PR to fix this issue. 

```
======================================================================
ERROR: test_ort_trainer (__main__.TestORTTrainer) (model_name='gpt2', dataset_name='sst2', inference_with_ort=False)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test_onnxruntime_train.py", line 131, in test_ort_trainer
    train_result = trainer.train()
  File "/workspace/optimum/onnxruntime/trainer.py", line 349, in train
    return inner_training_loop(
  File "/workspace/optimum/onnxruntime/trainer.py", line 615, in _inner_training_loop
    tr_loss_step = self.training_step(model, inputs)
  File "/usr/local/lib/python3.8/dist-packages/transformers/trainer.py", line 2523, in training_step
    loss = self.compute_loss(model, inputs)
  File "/usr/local/lib/python3.8/dist-packages/transformers/trainer.py", line 2555, in compute_loss
    outputs = model(**inputs)
  File "/usr/local/lib/python3.8/dist-packages/torch/nn/modules/module.py", line 1130, in _call_impl
    return forward_call(*input, **kwargs)
  File "/usr/local/lib/python3.8/dist-packages/onnxruntime/training/ortmodule/_utils.py", line 371, in _forward
    return ortmodule._torch_module.forward(*inputs, **kwargs)
  File "/usr/local/lib/python3.8/dist-packages/onnxruntime/training/ortmodule/_utils.py", line 351, in _forward
    return torch_module_ort._execution_manager(torch_module_ort.is_training()).forward(*inputs, **kwargs)
  File "/usr/local/lib/python3.8/dist-packages/onnxruntime/training/ortmodule/_training_manager.py", line 273, in forward
    self._fallback_manager.handle_exception(
  File "/usr/local/lib/python3.8/dist-packages/onnxruntime/training/ortmodule/_fallback.py", line 162, in handle_exception
    raise exception
  File "/usr/local/lib/python3.8/dist-packages/onnxruntime/training/ortmodule/_training_manager.py", line 210, in forward
    self._initialize_graph_builder()
  File "/usr/local/lib/python3.8/dist-packages/onnxruntime/training/ortmodule/_graph_execution_manager.py", line 478, in _initialize_graph_builder
    self._graph_builder.initialize(self._onnx_models.exported_model.SerializeToString(), grad_builder_config)
RuntimeError: /onnxruntime_src/orttraining/orttraining/python/orttraining_pybind_state.cc:731 onnxruntime::python::addObjectMethodsForTraining(pybind11::module&, onnxruntime::python::ExecutionProviderRegistrationFn)::<lambda(onnxruntime::training::OrtModuleGraphBuilder*, const pybind11::bytes&, const onnxruntime::training::OrtModuleGraphBuilderConfiguration&)> [ONNXRuntimeError] : 1 : FAIL : Type Error: Type parameter (T) of Optype (Where) bound to different types (tensor(float) and tensor(float16) in node (Where_223).
```