Change default dtype to bf16 (#39)

Signed-off-by: kaixuanliu <kaixuan.liu@intel.com>

Author: kaixuanliu

backends/src/dtype.rs

@@ -52,7 +52,7 @@ impl Default for DType {
             feature = "ort"
         )))]
         {
-            DType::Float16
+            DType::BFloat16
         }
     }
 }