fix(mla): rename _forward_decode to forward_mqa for vllm 0.20.0

vllm 0.20.0 renamed the MLACommonImpl decode override from _forward_decode
to forward_mqa (same signature). MLAFLImpl still defined _forward_decode, so
its implementation was never dispatched and the abstract forward_mqa raised
NotImplementedError at decode. Rename to match the base class.

Author: loopyt

vllm_fl/dispatch/backends/flaggems/impl/mla.py

@@ -119,7 +119,7 @@ def _flash_attn_varlen_diff_headdims(
             return attn_out, lse
         return attn_out
 
-    def _forward_decode(
+    def forward_mqa(
         self,
         q: Union[torch.Tensor, tuple[torch.Tensor, torch.Tensor]],
         kv_c_and_k_pe_cache: torch.Tensor,