Add abbreviation for bfloat16 data type in mfma_macro_generator.py

- Introduced a new abbreviation "bf16" for the bfloat16 data type in the mfma_macro_generator.py file, enhancing clarity and consistency in data type representation.

Author: LeiWang1999

tilelang/intrinsics/mfma_macro_generator.py

@@ -139,6 +139,7 @@ def _initialize_mfma_prefix(self, k_dim=16):
         }[out_dtype]
 
         in_dtype_abbrv = {
+            "bfloat16": "bf16",
             "float16": "f16",
             "float32": "f32",
             "int8": "i8",