fix: More fix of ml_dtypes<0.5 (#222)

This is a followup of #198 of a missing fix.

Author: oraluben

python/tvm_ffi/_dtype.py

@@ -314,7 +314,8 @@ def lanes(self) -> int:
     dtype._NUMPY_DTYPE_TO_STR[np.dtype(ml_dtypes.bfloat16)] = "bfloat16"
     dtype._NUMPY_DTYPE_TO_STR[np.dtype(ml_dtypes.float8_e4m3fn)] = "float8_e4m3fn"
     dtype._NUMPY_DTYPE_TO_STR[np.dtype(ml_dtypes.float8_e5m2)] = "float8_e5m2"
-    dtype._NUMPY_DTYPE_TO_STR[np.dtype(ml_dtypes.float4_e2m1fn)] = "float4_e2m1fn"
+    if hasattr(ml_dtypes, "float4_e2m1fn"):  # ml_dtypes >= 0.5.0
+        dtype._NUMPY_DTYPE_TO_STR[np.dtype(ml_dtypes.float4_e2m1fn)] = "float4_e2m1fn"
 except ImportError:
     pass