Mark bfloat16 as trainable.

PiperOrigin-RevId: 303816514
Change-Id: Iebaed635ea7adaf8ded604aba4d81907bce86395
diff --git a/tensorflow/python/eager/backprop_util.py b/tensorflow/python/eager/backprop_util.py
index ae026c0..117b05e 100644
--- a/tensorflow/python/eager/backprop_util.py
+++ b/tensorflow/python/eager/backprop_util.py
@@ -30,4 +30,4 @@
   dtype = dtypes.as_dtype(dtype)
   return dtype.base_dtype in (dtypes.float16, dtypes.float32, dtypes.float64,
                               dtypes.complex64, dtypes.complex128,
-                              dtypes.resource, dtypes.variant)
+                              dtypes.resource, dtypes.variant, dtypes.bfloat16)