[FEA] Support Series.value_counts

Groupby value_counts fails on when selecting individual columns from a DataFrame, but succeeds when running on the entire DataFrame.

```python
import pandas as pd
import cudf

gdf = cudf.datasets.randomdata(dtypes={"id": int, "x": int})
pdf = gdf.to_pandas()

print(pdf.groupby("id").x.value_counts().head())
print(gdf.groupby("id").x.value_counts())
id x
942 988 1
961 1026 1
965 1062 1
984 981 1
993 999 1
Name: count, dtype: int64
---------------------------------------------------------------------------
KeyError Traceback (most recent call last)
File [/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/cudf/core/groupby/groupby.py:2783](http://10.136.7.109:8881/lab/tree/nvme/0/nicholasb/benchmarks/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/cudf/core/groupby/groupby.py#line=2782), in _Grouping._handle_by_or_level(self, by, level)
2782 try:
-> 2783 self._handle_label(by)
2784 except (KeyError, TypeError):

File [/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/cudf/core/groupby/groupby.py:2845](http://10.136.7.109:8881/lab/tree/nvme/0/nicholasb/benchmarks/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/cudf/core/groupby/groupby.py#line=2844), in _Grouping._handle_label(self, by)
2844 else:
-> 2845 raise e
2846 self.names.append(by)

File [/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/cudf/core/groupby/groupby.py:2839](http://10.136.7.109:8881/lab/tree/nvme/0/nicholasb/benchmarks/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/cudf/core/groupby/groupby.py#line=2838), in _Grouping._handle_label(self, by)
2838 try:
-> 2839 self._key_columns.append(self._obj._data[by])
2840 except KeyError as e:
2841 # `by` can be index name(label) too.

File [/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/cudf/core/column_accessor.py:155](http://10.136.7.109:8881/lab/tree/nvme/0/nicholasb/benchmarks/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/cudf/core/column_accessor.py#line=154), in ColumnAccessor.__getitem__(self, key)
154 def __getitem__(self, key: Any) -> ColumnBase:
--> 155 return self._data[key]

KeyError: 'id'

During handling of the above exception, another exception occurred:

ValueError Traceback (most recent call last)
Cell In[31], line 8
5 pdf = gdf.to_pandas()
7 print(pdf.groupby("id").x.value_counts().head())
----> 8 print(gdf.groupby("id").x.value_counts())

File [/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/cudf/core/groupby/groupby.py:2598](http://10.136.7.109:8881/lab/tree/nvme/0/nicholasb/benchmarks/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/cudf/core/groupby/groupby.py#line=2597), in GroupBy.value_counts(self, subset, normalize, sort, ascending, dropna)
2591 raise ValueError(
2592 f"Keys {set(subset) & set(groupings)} in subset "
2593 "cannot be in the groupby column keys."
2594 )
2596 df["__placeholder"] = 1
2597 result = (
-> 2598 df.groupby(groupings + list(subset), dropna=dropna)[
2599 "__placeholder"
2600 ]
2601 .count()
2602 .sort_index()
2603 .astype(np.int64)
2604 )
2606 if normalize:
2607 levels = list(range(len(groupings), result.index.nlevels))

File [/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/nvtx/nvtx.py:116](http://10.136.7.109:8881/lab/tree/nvme/0/nicholasb/benchmarks/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/nvtx/nvtx.py#line=115), in annotate.__call__..inner(*args, **kwargs)
113 @wraps(func)
114 def inner(*args, **kwargs):
115 libnvtx_push_range(self.attributes, self.domain.handle)
--> 116 result = func(*args, **kwargs)
117 libnvtx_pop_range(self.domain.handle)
118 return result

File [/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/cudf/core/series.py:3426](http://10.136.7.109:8881/lab/tree/nvme/0/nicholasb/benchmarks/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/cudf/core/series.py#line=3425), in Series.groupby(self, by, axis, level, as_index, sort, group_keys, squeeze, observed, dropna)
3400 @_cudf_nvtx_annotate
3401 @docutils.doc_apply(
3402 groupby_doc_template.format(
(...)
3424 dropna=True,
3425 ):
-> 3426 return super().groupby(
3427 by,
3428 axis,
3429 level,
3430 as_index,
3431 sort,
3432 group_keys,
3433 squeeze,
3434 observed,
3435 dropna,
3436 )

File [/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/nvtx/nvtx.py:116](http://10.136.7.109:8881/lab/tree/nvme/0/nicholasb/benchmarks/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/nvtx/nvtx.py#line=115), in annotate.__call__..inner(*args, **kwargs)
113 @wraps(func)
114 def inner(*args, **kwargs):
115 libnvtx_push_range(self.attributes, self.domain.handle)
--> 116 result = func(*args, **kwargs)
117 libnvtx_pop_range(self.domain.handle)
118 return result

File [/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/cudf/core/indexed_frame.py:5337](http://10.136.7.109:8881/lab/tree/nvme/0/nicholasb/benchmarks/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/cudf/core/indexed_frame.py#line=5336), in IndexedFrame.groupby(self, by, axis, level, as_index, sort, group_keys, squeeze, observed, dropna)
5331 if group_keys is None:
5332 group_keys = False
5334 return (
5335 self.__class__._resampler(self, by=by)
5336 if isinstance(by, cudf.Grouper) and by.freq
-> 5337 else self.__class__._groupby(
5338 self,
5339 by=by,
5340 level=level,
5341 as_index=as_index,
5342 dropna=dropna,
5343 sort=sort,
5344 group_keys=group_keys,
5345 )
5346 )

File [/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/cudf/core/groupby/groupby.py:283](http://10.136.7.109:8881/lab/tree/nvme/0/nicholasb/benchmarks/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/cudf/core/groupby/groupby.py#line=282), in GroupBy.__init__(self, obj, by, level, sort, as_index, dropna, group_keys)
281 self.grouping = self._by
282 else:
--> 283 self.grouping = _Grouping(obj, self._by, level)

File [/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/cudf/core/groupby/groupby.py:2751](http://10.136.7.109:8881/lab/tree/nvme/0/nicholasb/benchmarks/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/cudf/core/groupby/groupby.py#line=2750), in _Grouping.__init__(self, obj, by, level)
2748 # Need to keep track of named key columns
2749 # to support `as_index=False` correctly
2750 self._named_columns = []
-> 2751 self._handle_by_or_level(by, level)
2753 if len(obj) and not len(self._key_columns):
2754 raise ValueError("No group keys passed")

File [/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/cudf/core/groupby/groupby.py:2785](http://10.136.7.109:8881/lab/tree/nvme/0/nicholasb/benchmarks/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/cudf/core/groupby/groupby.py#line=2784), in _Grouping._handle_by_or_level(self, by, level)
2783 self._handle_label(by)
2784 except (KeyError, TypeError):
-> 2785 self._handle_misc(by)

File [/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/cudf/core/groupby/groupby.py:2868](http://10.136.7.109:8881/lab/tree/nvme/0/nicholasb/benchmarks/nvme/0/nicholasb/miniconda3/envs/rapids-24.06/lib/python3.10/site-packages/cudf/core/groupby/groupby.py#line=2867), in _Grouping._handle_misc(self, by)
2866 by = cudf.core.column.as_column(by)
2867 if len(by) != len(self._obj):
-> 2868 raise ValueError("Grouper and object must have same length")
2869 self._key_columns.append(by)
2870 self.names.append(None)

ValueError: Grouper and object must have same length
```

```python
print(gdf.groupby("id").value_counts()) # succeeds
# print(gdf.groupby("id")[["x"]].value_counts()) # same error as above
```