fix(tuners): guard child.weight access in _replace_module for non-linear modules

[Chessing234]

Bug: _replace_module in tuners_utils.py unconditionally assigns new_module.weight = child.weight without checking whether child actually has a weight attribute:

if not hasattr(new_module, "base_layer"):
    new_module.weight = child.weight   # crashes if child has no weight
    if hasattr(child, "bias"):
        new_module.bias = child.bias

When modules_to_save contains non-linear module blocks (e.g. Siglip2EncoderLayer, full transformer encoder layers, or any module without a weight parameter), calling merge_and_unload() raises:

AttributeError: 'Siglip2EncoderLayer' object has no attribute 'weight'

Root cause: The bias assignment already uses hasattr(child, "bias") as a guard, but the weight assignment has no equivalent guard. Non-linear modules in modules_to_save legitimately lack weight.

Fix: Add the same hasattr guard for weight that already exists for bias:

if not hasattr(new_module, "base_layer"):
    if hasattr(child, "weight"):
        new_module.weight = child.weight
    if hasattr(child, "bias"):
        new_module.bias = child.bias

This is a one-line addition matching the existing convention in the same block.

[BenjaminBossan]

Thanks for providing this PR. It would be great to add a unit test to catch this error.