[300I][Bugfix] fix unquant model weight nd2nz error (#6851)

### What this PR does / why we need it?
- This PR fixes an issue with weight format conversion for unquantized
models running on Ascend 310P devices.

- The changes refactor the logic for converting weights to the
FRACTAL_NZ format. Previously, this was handled in a 310P-specific
linear layer implementation (`AscendUnquantizedLinearMethod310`). This
implementation has been removed, and the logic is now centralized in the
`maybe_trans_nz` utility function. This function now checks if the
device is a 310P and applies the NZ format cast accordingly for
`float16`/`bfloat16` weights.

- This refactoring simplifies the code by removing platform-specific
duplication and ensures correct weight handling for unquantized models
on 310P.

### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
ut and local test
- vLLM version: v0.15.0
- vLLM main:
83b47f67b1

---------

Signed-off-by: Tflowers-0129 <2906339855@qq.com>

tests/ut/_310p/quantization/test_modelslim_config_310.py

@@ -21,7 +21,7 @@ from vllm.model_executor.layers.linear import LinearBase
 
 from tests.ut.base import TestBase
 from vllm_ascend._310p.fused_moe.fused_moe import AscendUnquantizedFusedMoEMethod310
-from vllm_ascend._310p.ops.linear import AscendUnquantizedLinearMethod310
+from vllm_ascend.ops.linear import AscendUnquantizedLinearMethod
 from vllm_ascend._310p.quantization.modelslim_config import AscendModelSlimConfig310
 
 
@@ -50,7 +50,7 @@ class TestAscendModelSlimConfig310(TestBase):
             patch.object(self.ascend_config, "is_layer_skipped_ascend", return_value=True),
         ):
             method = self.ascend_config.get_quant_method(linear_layer, ".attn")
-            self.assertIsInstance(method, AscendUnquantizedLinearMethod310)
+            self.assertIsInstance(method, AscendUnquantizedLinearMethod)
 
         # Test quantized layer
         mock_scheme = MagicMock()