Add tests for issue #1074 and update Categorify args

Add tests for issue #1074 on adding a start_index argument to
the Categorify op. The motivation for this issue is to allow an
offset for encoding out-of-vocabulary and other special values.

In this commit, we add a test function to test various values of our
new start_index arg in tests/unit/ops/text_ops.py and add a
start_value arg to the class signature for Categorify(),
documentation for its intended function in Categorify()'s
docstring, a start_value arg to FitOptions(), and documentation
for this new argument in FitOptions()'s docstring.

nvtabular/ops/categorify.py

@@ -174,6 +174,12 @@ class Categorify(StatOperator):
         value will be `max_size - num_buckets -1`.  Setting the max_size param means that
         freq_threshold should not be given.  If the num_buckets parameter is set,  it must be
         smaller than the max_size value.
+    start_index: int, default 0
+        The start index where Categorify will begin to translate dataframe entries
+        into integer values. For instance, if our original translated dataframe entries appear 
+        as [[1], [1, 4], [3, 2], [2]], then with a start_index of 16, Categorify will now be 
+        [[16], [16, 19], [18, 17], [17]]. This option is useful to reserve an intial segment 
+        of non-negative translated integers for out-of-vocabulary or other special values. 
     """
 
     def __init__(
@@ -191,6 +197,7 @@ def __init__(
         num_buckets=None,
         vocabs=None,
         max_size=0,
+        start_index=0,
     ):
 
         # We need to handle three types of encoding here:
@@ -603,6 +610,8 @@ class FitOptions:
         num_buckets:
             If specified will also do hashing operation for values that would otherwise be mapped
             to as unknown (by freq_limit or max_size parameters)
+        start_index: int
+            The index to start mapping our output categorical values to. 
     """
 
     col_groups: list
@@ -617,6 +626,7 @@ class FitOptions:
     name_sep: str = "-"
     max_size: Optional[Union[int, dict]] = None
     num_buckets: Optional[Union[int, dict]] = None
+    start_index: int = 0
 
     def __post_init__(self):
         if not isinstance(self.col_groups, ColumnSelector):

tests/unit/ops/test_ops.py

@@ -504,6 +504,41 @@ def test_categorify_lists(tmpdir, freq_threshold, cpu, dtype, vocabs):
     else:
         assert compare == [[1], [1, 0], [0, 2], [2]]
 
+@pytest.mark.parametrize("cpu", _CPU)
+@pytest.mark.parametrize("dtype", [None, np.int32, np.int64])
+@pytest.mark.parametrize("vocabs", [None, pd.DataFrame({"Authors": [f"User_{x}" for x in "ACBE"]})])
+@pytest.mark.parametrize("start_index", [0, 2, 16])
+def test_categorify_lists_with_start_index(tmpdir, cpu, dtype, vocabs, start_index):
+    df = dispatch._make_df(
+        {
+            "Authors": [["User_A"], ["User_A", "User_E"], ["User_B", "User_C"], ["User_C"]],
+            "Engaging User": ["User_B", "User_B", "User_A", "User_D"],
+            "Post": [1, 2, 3, 4],
+        }
+    )
+    cat_names = ["Authors", "Engaging User"]
+    label_name = ["Post"]
+
+    cat_features = cat_names >> ops.Categorify(
+        out_path=str(tmpdir), dtype=dtype, vocabs=vocabs
+    )
+
+    workflow = nvt.Workflow(cat_features + label_name)
+    df_out = workflow.fit_transform(nvt.Dataset(df, cpu=cpu)).to_ddf().compute()
+
+    if cpu:
+        compare = [list(row) for row in df_out["Authors"].tolist()]
+    else:
+        compare = df_out["Authors"].to_arrow().to_pylist()
+
+    if start_index == 0:
+        assert compare == [[1], [1, 4], [3, 2], [2]]
+
+    if start_index == 2:
+        assert compare == [[3], [3, 6], [5, 4], [4]]
+
+    if start_index == 16:
+        assert compare == [[16], [16, 19], [18, 17], [17]]
 
 @pytest.mark.parametrize("cat_names", [[["Author", "Engaging User"]], ["Author", "Engaging User"]])
 @pytest.mark.parametrize("kind", ["joint", "combo"])