[Refactor][Quantization] Reorganize quantization module: group configs and methods into subdirectories (#14030)

### What this PR does / why we need it?

- Part of https://github.com/vllm-project/vllm-ascend/issues/13318

**Summary**

This PR reorganizes the vllm_ascend.quantization module to improve code
organization and maintainability as the number of quantization configs
and methods continues to grow.

**Changes**

1. Directory Restructuring
    Configs (quantization/configs/):
- Moved 4 config files (compressed_tensors_config.py, fp8_config.py,
modelslim_config.py, modelopt_mxfp8_config.py) into dedicated configs/
subdirectory
      
Methods (quantization/methods/): Grouped 16 method files into 5
subdirectories by quantization type
- w8a8/ — W8A8 quantization (dynamic, static, mxfp8, pdmix, fp8_dynamic)
     - w4a8/ — W4A8 quantization (dynamic, mxfp4)
     - wna16/ — WnA16 quantization (w4a16, w4a16_mxfp4, w8a16)
- w4a4/ — W4A4 quantization (flatquant, laos_dynamic, mxfp4,
mxfp4_flatquant)
     - kv_cache/ — KV cache quantization (kv_c8)
- Eliminated fp8/ directory — merged 2 DS-specific classes into parent
files (w8a8_mxfp8.py, w4a8_mxfp4.py)

2. Dead Code Removal
- Remove unused attribute assignments: `self.ep_group`
`self.use_aclgraph` .etc
- Delete some meaningless code in `AscendCompressedTensorsConfig`
3. Code Optimization
- Extracted shared utility: Moved duplicate _is_fused_moe_layer()
function (4 copies) to quantization/utils.py as is_fused_moe_layer()
- Extracted shared solver: Moved duplicate solve_kronecker_decompose()
to w4a4_flatquant.py, shared with w4a4_mxfp4_flatquant.py
- Remove `layer.ascend_quant_method`

### Does this PR introduce _any_ user-facing change?
no

### How was this patch tested?

CI pass


- vLLM main:
https://github.com/vllm-project/vllm/commit/ba07e4a48fc951300d97eb506217dd530583dea3

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Signed-off-by: wangkunpeng <1289706727@qq.com>