[BugFix][TritonMLA] Process weights after model loading for GGUF

[tywuAMD]

Issue Description
Observed the following error message when trying to deploy DeepSeek R1 Unsloth GGUF quantized model with MLA enabled on Triton MLA backend:

ERROR 03-10 04:33:58 [multiproc_worker_utils.py:242]   File "vllm/attention/layer.py", line 329, in unified_attention
ERROR 03-10 04:33:58 [multiproc_worker_utils.py:242]     return self.impl.forward(self, query, key, value, kv_cache, attn_metadata)
ERROR 03-10 04:33:58 [multiproc_worker_utils.py:242]   File "vllm/attention/backends/mla/common.py", line 1537, in forward
ERROR 03-10 04:33:58 [multiproc_worker_utils.py:242]     decode_q_nope = self._q_proj_and_k_up_proj(decode_hs_or_q_c)
ERROR 03-10 04:33:58 [multiproc_worker_utils.py:242]   File "vllm/attention/backends/mla/common.py", line 1089, in _q_proj_and_k_up_proj
ERROR 03-10 04:33:58 [multiproc_worker_utils.py:242]     if is_fp8(self.W_Q_UK):
ERROR 03-10 04:33:58 [multiproc_worker_utils.py:242] AttributeError: 'TritonMLAImpl' object has no attribute 'W_Q_UK'

Fix
It turns out that W_Q_UK never got initialized. To fix it, call _process_weights_after_loading() right after model loading to properly process the weight attributes.

Local Verification
With this PR, the mentioned error message cannot be reproduced anymore, and the DeepSeek GGUF model can be deployed successfully.

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[cjackal]

Don't know why but bot failed to ping vLLM maintainers; I guess @Isotr0py is the right person here.

[mgoin]

LGTM!