CLN: de-duplicate generate_range
Split off from #23140.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23218?src=pr&el=h1) Report
> Merging [#23218](https://codecov.io/gh/pandas-dev/pandas/pull/23218?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/145c2275e3560edc30ff418a57df67ba3c4c30d6?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23218/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23218?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23218      +/-   ##
==========================================
+ Coverage   92.19%    92.2%   +<.01%     
==========================================
  Files         169      169              
  Lines       50972    50968       -4     
==========================================
- Hits        46996    46993       -3     
+ Misses       3976     3975       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.62% <100%> (ø)` | :arrow_up: |
| #single | `42.26% <81.25%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23218?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/23218/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lbGlrZS5weQ==) | `94.88% <ø> (-0.02%)` | :arrow_down: |
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/23218/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `93.22% <ø> (-0.02%)` | :arrow_down: |
| [pandas/core/arrays/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/23218/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3BlcmlvZC5weQ==) | `95.58% <100%> (+0.01%)` | :arrow_up: |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/23218/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `98.27% <100%> (+0.02%)` | :arrow_up: |
| [pandas/core/arrays/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/23218/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3RpbWVkZWx0YXMucHk=) | `94.47% <100%> (+0.5%)` | :arrow_up: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/23218/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `96.47% <100%> (-0.03%)` | :arrow_down: |
| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/23218/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `90.62% <100%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23218?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23218?src=pr&el=footer). Last update [145c227...a9c021a](https://codecov.io/gh/pandas-dev/pandas/pull/23218?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 > no what I would do is add this to DatetimeIndex. [...]

Um, I don't see how that is really relevant to the `_validate_frequency` usage.  `DatetimeArray._generate_range` already accepts `ambiguous`; it is a matter of passing that arg in `_validate_frequency`. I mean, I could just take out the new assertion in `_validate_frequency` and the method wouldn't be changed at all by this PR. > Um, I don't see how that is really relevant to the _validate_frequency usage. DatetimeArray._generate_range already accepts ambiguous; it is a matter of passing that arg in _validate_frequency.

then let's just make this the default. This is a very kludgy way of doing that. Hello @jbrockmendel! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/core/arrays/datetimelike.py`](https://github.com/jbrockmendel/pandas/blob/a9c021a4019dc449b44bd2e9104cb8e9f06914e1/pandas/core/arrays/datetimelike.py) !

 - There are no PEP8 issues in the file [`pandas/core/arrays/period.py`](https://github.com/jbrockmendel/pandas/blob/a9c021a4019dc449b44bd2e9104cb8e9f06914e1/pandas/core/arrays/period.py) !

 - There are no PEP8 issues in the file [`pandas/core/arrays/timedeltas.py`](https://github.com/jbrockmendel/pandas/blob/a9c021a4019dc449b44bd2e9104cb8e9f06914e1/pandas/core/arrays/timedeltas.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/datetimelike.py`](https://github.com/jbrockmendel/pandas/blob/a9c021a4019dc449b44bd2e9104cb8e9f06914e1/pandas/core/indexes/datetimelike.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/datetimes.py`](https://github.com/jbrockmendel/pandas/blob/a9c021a4019dc449b44bd2e9104cb8e9f06914e1/pandas/core/indexes/datetimes.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/period.py`](https://github.com/jbrockmendel/pandas/blob/a9c021a4019dc449b44bd2e9104cb8e9f06914e1/pandas/core/indexes/period.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/timedeltas.py`](https://github.com/jbrockmendel/pandas/blob/a9c021a4019dc449b44bd2e9104cb8e9f06914e1/pandas/core/indexes/timedeltas.py) !

 looks fine. not super happy about having to set name all over the place, but this is an improvement over before. merging after CI is fixed. thanks!