FEAT 5460: Make DataFrameGroupBy.value counts more distributed (#3)

* FIX-modin-project#4154: add value_counts method for SeriesGroupBy and DataFrameGroupBy (modin-project#5453)

Signed-off-by: Anatoly Myachev <anatoly.myachev@intel.com>

* WIP GroupBy

* Seemingly working w/ Ponder and Modin

* Maintain proper value ordering for single group-by operations

* linter updates from black on the files that have changed

* When normalize is used, or the groupby was performed with as_index=False, default to pandas.
With ponder this will result in a NotImplemented error currently.

* Remove _to_pandas() by implementing sort_index on Series on the service side

* | Object | call | Pushdown? | Compatibility |
| --- | --- | --- | --- |
| DataFrameGroupBy | value_counts() | Full | PASS |
| DataFrameGroupBy | value_counts(ascending=True) |Full | PASS |
| DataFrameGroupBy | value_counts(ascending=False) | Full |PASS |
| DataFrameGroupBy | value_counts(sort=False) |Full | PASS |
| DataFrameGroupBy | value_counts(sort=True) |Full | PASS |
| DataFrameGroupBy | value_counts(normalize=False) |Full | PASS |
| DataFrameGroupBy | value_counts(normalize=True) | | FAIL |
| DataFrame | groupby(as_index=False) |  | FAIL |
| DataFrameGroupBy | value_counts(dropna=False) | Full  | PASS
| DataFrameGroupBy | value_counts(dropna=True) |  |FAIL

$\color{red}{\text{NOTE: For MultiIndex GroupBys the n+1 level index is ignored in the sorting.}}$

---------

Signed-off-by: Anatoly Myachev <anatoly.myachev@intel.com>
Co-authored-by: Anatoly Myachev <anatoly.myachev@intel.com>

modin/pandas/groupby.py

@@ -218,7 +218,7 @@ def skew(self, *args, **kwargs):
         )
 
     def ffill(self, limit=None):
-        return self.fillna(limit=limit, method='ffill')
+        return self.fillna(limit=limit, method="ffill")
 
     def sem(self, ddof=1):
         return self._wrap_aggregation(
@@ -235,14 +235,35 @@ def value_counts(
         ascending: bool = False,
         dropna: bool = True,
     ):
-        return self._default_to_pandas(
-            lambda df: df.value_counts(
+        # Compatibility Notes:
+        # dfGroupBy.value_counts nearly semantically
+        # equivalent to df.value_counts([<by>, <other...>]).sort_index()
+        # it returns a MultiIndex Series which needs to be converted to
+        # pandas for sort_index.
+        # 
+        # Semantic Exceptions:
+        # normalize does not work; it will return the normalized results
+        #     across the entire dataframe, not within the sub levels
+        # DataFrame(as_index=False) does not work. The default is True
+        #     calling this function will always result in a Series rather 
+        #     than a DataFrame
+        #
+        if is_list_like(self._by):
+            subset = self._by
+        elif isinstance(self._by, type(self._query_compiler)):
+            subset = self._by.columns.values.tolist()
+        for c in self._columns.values.tolist():
+            if c not in subset:
+                subset.append(c)
+        return (
+            self._df.value_counts(
                 subset=subset,
                 normalize=normalize,
                 sort=sort,
                 ascending=ascending,
                 dropna=dropna,
             )
+            .sort_index(level=0, sort_remaining=False)
         )
 
     def mean(self, numeric_only=None):
@@ -618,7 +639,7 @@ def cummin(self, axis=0, **kwargs):
         )
 
     def bfill(self, limit=None):
-        return self.fillna(limit=limit, method='bfill')
+        return self.fillna(limit=limit, method="bfill")
 
     def idxmin(self):
         return self._default_to_pandas(lambda df: df.idxmin())
@@ -787,7 +808,7 @@ def corrwith(self):
         return self._default_to_pandas(lambda df: df.corrwith)
 
     def pad(self, limit=None):
-        return self.fillna(limit=limit, method='pad')
+        return self.fillna(limit=limit, method="pad")
 
     def max(self, numeric_only=False, min_count=-1):
         return self._wrap_aggregation(
@@ -1453,6 +1474,24 @@ def _iter(self):
                 for k in (sorted(group_ids) if self._sort else group_ids)
             )
 
+    def value_counts(
+        self,
+        normalize: bool = False,
+        sort: bool = True,
+        ascending: bool = False,
+        bins=None,
+        dropna: bool = True,
+    ):
+        return self._default_to_pandas(
+            lambda ser: ser.value_counts(
+                normalize=normalize,
+                sort=sort,
+                ascending=ascending,
+                bins=bins,
+                dropna=dropna,
+            )
+        )
+
     def unique(self):
         return self._check_index(
             self._wrap_aggregation(