metal : fix `ggml_metal_supports_op` for BF16 (#8021)

Currently the Metal backend does not support BF16. `ggml_metal_supports_op` was returning true in these cases, leading to a crash with models converted with `--leave-output-tensor`. This commit checks if the first few sources types are BF16 and returns false if that's the case.

ggml-metal.m

@@ -735,6 +735,12 @@ static id<MTLBuffer> ggml_metal_get_buffer(struct ggml_tensor * t, size_t * offs
 }
 
 static bool ggml_metal_supports_op(const struct ggml_metal_context * ctx, const struct ggml_tensor * op) {
+    for (size_t i = 0, n = 3; i < n; ++i) {
+        if (op->src[i] != NULL && op->src[i]->type == GGML_TYPE_BF16) {
+            return false;
+        }
+    }
+
     switch (op->op) {
         case GGML_OP_UNARY:
             switch (ggml_get_unary_op(op)) {