metal : optimize FA vec for large sequences and BS <= 8 (#15566)

* metal : optmize FA vec for large heads and sequences

* metal : adjust small-batch mul mv kernels

ggml-ci

* batched-bench : fix total speed computation

ggml-ci

* cont : add comments

ggml-ci

ggml/src/ggml-metal/ggml-metal-impl.h

@@ -249,6 +249,7 @@ typedef struct {
     uint64_t nb33;
     int32_t  ne1;
     int32_t  ne2;
+    int32_t  ne3;
     float    scale;
     float    max_bias;
     float    m0;
@@ -257,6 +258,11 @@ typedef struct {
     float    logit_softcap;
 } ggml_metal_kargs_flash_attn_ext;
 
+typedef struct {
+    int32_t  nrows;
+    int32_t  ne20;
+} ggml_metal_kargs_flash_attn_ext_reduce;
+
 typedef struct {
     int32_t  ne00;
     int32_t  ne02;