[BUG] Some pandas code-flows still return `ns` time resolution

**Describe the bug**
Some pandas `list-like` inputs with datetime data still need to return `ns` as opposed to type-efficient resolutions like `s`, `ms` and `us` that arrow and cudf does. This is the cause for failures in: tests/indexing/test_loc.py::TestLocSetitemWithExpansion::test_loc_setitem_datetime_keys_cast

**Steps/Code to reproduce bug**
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**Expected behavior**
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**Environment overview (please complete the following information)**
- Environment location: [Bare-metal, Docker, Cloud(specify cloud provider)]
- Method of cuDF install: [conda, Docker, or from source]
- If method of install is [Docker], provide `docker pull` & `docker run` commands used

**Environment details**
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**Additional context**
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