Allowing %f in format to return nanoseconds

[marlenezw]

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.

[marlenezw]

rerun tests

[codecov]

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[cwharris]

rerun tests

[shwina]

@gpucibot merge