fix error for _register_pytree_node in torch2.1.0 and  fix bf16 assertion in xpu and npu
Author: jiaqiw09
Pull request: yes

# What does this PR do?

fix issue #37838

## fix error for _register_pytree_node

The PR [Apply torchfix to replace deprecated functions: `_pytree._register_pytree_node` and `torch.cpu.amp.autocast`](https://github.com/huggingface/transformers/commit/71b35387fd6d71487bd29e694ed10d925203e031) introduced a compatibility issue when using **Torch 2.1.0**.

Specifically, in **Torch 2.1.0**, the correct import should still be:

```python
from torch.utils._pytree import _register_pytree_node
```

However, the PR incorrectly tries to import:

```python
from torch.utils._pytree import register_pytree_node
```

This leads to the following error when running with **Torch 2.1.0**:

```
ImportError: cannot import name 'register_pytree_node' from 'torch.utils._pytree'
```

## fix bf16 assertion in xpu and npu

The PR [Add XPU case to `is_torch_bf16_gpu_available`](https://github.com/huggingface/transformers/commit/954f31cd818c431312f452c4e10bcbc0bdde42a2) introduced a compatibility issue:

- It **only handled XPU** and **ignored other devices** like NPU.
- It **also triggered an error** when XPU is **not available** in the current PyTorch build.
  - Directly accessing `torch.xpu.is_available()` without checking if `torch.xpu` exists causes an `AttributeError`.

When `torch.xpu` is not present (which is common in standard PyTorch installations), the following code pattern:

```python
if torch.xpu.is_available():
    ...
```

causes:

```
AttributeError: module 'torch' has no attribute 'xpu'
```

We use a safer access pattern:

```python
if hasattr(torch, "xpu") and torch.xpu.is_available():
    ...
```

This ensures compatibility even when `torch.xpu` is not defined.

In addition, I have also added support for NPU devices by checking:

```python
if is_torch_npu_available():
    ...
```

## Before submitting
- [x] 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#create-a-pull-request), Pull Request section?
- [ ] 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.
- [x] 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?

- trainer: @zach-huggingface and @SunMarc

Comments:
Hi 👋, thank you for opening this pull request! The pull request is converted to draft by default. The CI will be paused while the PR is in draft mode. When it is ready for review, please click the `Ready for review` button (at the bottom of the PR page). This will assign reviewers and trigger CI.

It seems to impact the usage of transformers on Ascend NPU with torch==2.1.0, please help us review and merge, thanks 😃 @ArthurZucker @Rocketknight1  

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