Zen4 Flash Attention - bf16 support (#38)

* Zen4 Flash Attnetion: WIP bf16

* Zen4 Flash Attnetion: bf16 seems to be working

* Zen4 Flash Attnetion: improving bf16

* Zen4 Flash Attnetion: improving bf16

It is better (slightly faster) to first convert Q
to bf16 before processing each block of q_step rows.
This requires D*q_step*sizeof(bf16) bytes, so at
most 4 kb for the head sizes we support, so we can
just allocate on the stack instead of reserving and
passing a work buffer in ggml.

---------

Co-authored-by: Iwan Kawrakow <iwan.kawrakow@gmail.com>

common/common.cpp

@@ -2221,6 +2221,9 @@ static ggml_type kv_cache_type_from_str(const std::string & s) {
     if (s == "f16") {
         return GGML_TYPE_F16;
     }
+    if (s == "bf16") {
+        return GGML_TYPE_BF16;
+    }
     if (s == "q8_0") {
         return GGML_TYPE_Q8_0;
     }