[merger] fix: avoid setting torch's global device to meta

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What does this PR do?

This PR fixes several issues (#1484, #1255) that cause the error: "Cannot copy out of meta tensor; no data!".

The related code in our part is:
https://github.com/volcengine/verl/blob/d36b5e81d6de598a87eecc6edf658ece0eb43582/scripts/model_merger.py#L131-L132

The torch.device("meta") context manager sets the current global torch device to "meta". During auto_model_class.from_config, various import statements load third-party libraries, whose __init__.py files may contain global statements that use torch for calculations.

For example, transformers imports [torchao](https://github.com/pytorch/ao/blob/5549da8af975be6ff14330feb56c4abe3405b6f9/torchao/optim/subclass_4bit.py#L33), which executes the following during initialization:

QMAP_UNSIGNED = torch.linspace(0, 1, 17)[1:].tolist()  # no zero

In this case, when using the torch.device("meta") context manager, torch.linspace(0, 1, 17) gets created on the meta device, which only assigns metadata and cannot be moved to CPU. This causes the .tolist() call to fail with the error "Cannot copy out of meta tensor; no data!"

To fix this, we're now using init_empty_weights from accelerate, which patches nn.Module.register_parameter instead of patching torch's global device (https://github.com/huggingface/accelerate/blob/417bc529654a70e61013fd21263826a2f1f9e1a6/src/accelerate/big_modeling.py#L96-L170), thus avoiding this issue.

Here's a simple illustration:

>>> import torch
>>> from accelerate import init_empty_weights
>>> with init_empty_weights():
...     QMAP_UNSIGNED = torch.linspace(0, 1, 17)[1:].tolist()
... 
>>> QMAP_UNSIGNED
[0.0625, 0.125, 0.1875, 0.25, 0.3125, 0.375, 0.4375, 0.5, 0.5625, 0.625, 0.6875, 0.75, 0.8125, 0.875, 0.9375, 1.0]
>>> with torch.device("meta"):
...     QMAP_UNSIGNED = torch.linspace(0, 1, 17)[1:].tolist()
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "/usr/local/lib/python3.10/dist-packages/torch/utils/_device.py", line 104, in __torch_function__
    return func(*args, **kwargs)
NotImplementedError: Cannot copy out of meta tensor; no data!

cc @ETOgaosion

Additional Info.

• Issue Number: Fixes issue qwen3 merge_model error #1484, Cannot copy out of meta tensor in model_merger.py #1255, [model merger] refactor model merger for better usage and maintainability #1468 (comment)
• Training: both
• Inference: none

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