util: Explicitly call __dlpack__ built-in method in xp2tensorflow (#757)

`tf.experimental.dlpack.from_dlpack` expects a `PyCapsule` object.

Author: shadeMe

thinc/util.py

@@ -404,7 +404,8 @@ def xp2tensorflow(
         dlpack_tensor = xp_tensor.toDlpack()  # type: ignore
         tf_tensor = tf.experimental.dlpack.from_dlpack(dlpack_tensor)
     elif hasattr(xp_tensor, "__dlpack__"):
-        tf_tensor = tf.experimental.dlpack.from_dlpack(xp_tensor)
+        dlpack_tensor = xp_tensor.__dlpack__()  # type: ignore
+        tf_tensor = tf.experimental.dlpack.from_dlpack(dlpack_tensor)
     else:
         tf_tensor = tf.convert_to_tensor(xp_tensor)
     if as_variable: