Allowing %f in format to return nanoseconds (#9081)

This is a quick fix to close PR #7945 
This PR checks to see if `%f` is passed as part of `format`  into `cudf.to_datetime`. Previously, cudf would not return nanoseconds, while pandas does.

Authors:
  - Marlene  (https://github.com/marlenezw)

Approvers:
  - Ashwin Srinath (https://github.com/shwina)

URL: #9081

python/cudf/cudf/core/tools/datetimes.py

@@ -123,6 +123,9 @@ def to_datetime(
     if yearfirst:
         raise NotImplementedError("yearfirst support is not yet implemented")
 
+    if format is not None and "%f" in format:
+        format = format.replace("%f", "%9f")
+
     try:
         if isinstance(arg, cudf.DataFrame):
             # we require at least Ymd

python/cudf/cudf/tests/test_datetime.py

@@ -717,6 +717,7 @@ def test_to_datetime_units(data, unit):
         (["10/11/2012", "01/01/2010", "07/07/2016", "02/02/2014"], "%m/%d/%Y"),
         (["10/11/2012", "01/01/2010", "07/07/2016", "02/02/2014"], "%d/%m/%Y"),
         (["10/11/2012", "01/01/2010", "07/07/2016", "02/02/2014"], None),
+        (["2021-04-13 12:30:04.123456789"], "%Y-%m-%d %H:%M:%S.%f"),
         (pd.Series([2015, 2020, 2021]), "%Y"),
         pytest.param(
             pd.Series(["1", "2", "1"]),