Replace nvtabular inference back-end with python (#1771)

The NVTabular custom back-end is no longer maintained and shouldn't be used. The Triton Python back-end is now sufficient for what we need, so we can switch over to that.

Author: karlhigley

nvtabular/inference/triton/ensemble.py

@@ -36,7 +36,7 @@ def export_tensorflow_ensemble(
     label_columns=None,
     sparse_max=None,
     version=1,
-    nvtabular_backend="nvtabular",
+    nvtabular_backend="python",
     cats=None,
     conts=None,
 ):