Adds dbias accumulation flag to bwd params

Introduces a toggle to optionally accumulate bias gradients during backward attention.
Enables skipping unnecessary dbias work when unused and provides clearer control for kernels, aiding performance and configurability.

Author: algo-home

csrc/flash_dmattn/src/flash.h

@@ -195,6 +195,8 @@ struct Flash_bwd_params : public Flash_fwd_params {
 
     bool deterministic;
     index_t dq_accum_split_stride;
+
+    bool accum_dbias;
 };
 
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