BUG: DataFrameGroupBy.transform and ngroup do not work with cumcount
- [ ] closes #27472 
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @charlesdong1991! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:







There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-08-23 19:38:32 UTC yeah, you are right, pushing to my pr again will somehow pass flaky tests! @TomAugspurger  @jbrockmendel can you take a glance at this? will do is flaky test failure back 😢? also saw the same failure in other PR @TomAugspurger  Restarted the azure builds. Thanks Tom, looks okay except one thing: what is `codecov/project` about? how to fix such issue? @TomAugspurger 😅  You can ignore that.

On Thu, Aug 15, 2019 at 9:19 AM Kaiqi Dong <notifications@github.com> wrote:

> Thanks Tom, looks okay except one thing: what is codecov/project about?
> how to fix such issue? @TomAugspurger <https://github.com/TomAugspurger>
> 😅
>
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 any follow-up review? ^^ @TomAugspurger  probably this is not gonna be merged today, then shall I move whatsnew note to 1.0.0? @TomAugspurger  Yes, that's probably best. I haven't had a chance to go through it again.

On Wed, Aug 21, 2019 at 3:30 PM Kaiqi Dong <notifications@github.com> wrote:

> if this is not gonna be merged today, then shall I move whatsnew note to
> 1.0.0? @TomAugspurger <https://github.com/TomAugspurger>
>
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 ok, will move after the 0.25.1 releases Can you also move to v1.0.0? > Can you also move to v1.0.0?

yeah, of course.. was planning to do so also needs a whatsnew note (1.0, groupby bug fix) @charlesdong1991 can you merge master and update to any open comments @charlesdong1991 can you rebase