metal : add BF16 support (#8439)

* ggml : add initial BF16 support

ggml-ci

* metal : add mul_mat_id BF16 support

ggml-ci

* metal : check for bfloat support on the Metal device

ggml-ci

* metal : better var names [no ci]

* metal : do not build bfloat kernels when not supported

ggml-ci

* metal : try to fix BF16 support check

ggml-ci

* metal : this should correctly check bfloat support

common/common.cpp

@@ -1003,6 +1003,9 @@ 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;
     }