Fix API docs (#1821)

Author: mikemckiernan

tox.ini

@@ -87,6 +87,7 @@ commands =
 changedir = {toxinidir}
 deps = -rrequirements/docs.txt
 commands =
+    python -m pip install --upgrade git+https://github.com/NVIDIA-Merlin/core.git
     sphinx-multiversion --dump-metadata docs/source docs/build/html | jq "keys"
     sphinx-multiversion docs/source docs/build/html