[DOCS] Changing docs for UDF (#2880)

1. Improves UDF documentation by adding API pages for `StatefulUDF` and
`StatelessUDF`, with lots of docstrings and examples
~2. Moves our `daft.udf` module to `daft.udfs` instead, which avoids a
naming conflict with `daft.udf` which is our decorator. This might be a
breaking change...~ (Reverted)

---------

Co-authored-by: Jay Chia <[email protected]@users.noreply.github.com>

daft/dataframe/dataframe.py

@@ -3083,10 +3083,13 @@ def map_groups(self, udf: Expression) -> "DataFrame":
 
         Example:
             >>> import daft, statistics
+            >>>
             >>> df = daft.from_pydict({"group": ["a", "a", "a", "b", "b", "b"], "data": [1, 20, 30, 4, 50, 600]})
+            >>>
             >>> @daft.udf(return_dtype=daft.DataType.float64())
             ... def std_dev(data):
             ...     return [statistics.stdev(data.to_pylist())]
+            >>>
             >>> df = df.groupby("group").map_groups(std_dev(df["data"]))
             >>> df.show()
             ╭───────┬────────────────────╮

daft/expressions/expressions.py

@@ -1001,7 +1001,7 @@ def apply(self, func: Callable, return_dtype: DataType) -> Expression:
         Returns:
             Expression: New expression after having run the function on the expression
         """
-        from daft.udf import StatelessUDF
+        from daft.udf import CommonUDFArgs, StatelessUDF
 
         def batch_func(self_series):
             return [func(x) for x in self_series.to_pylist()]
@@ -1015,8 +1015,10 @@ def batch_func(self_series):
             name=name,
             func=batch_func,
             return_dtype=return_dtype,
-            resource_request=None,
-            batch_size=None,
+            common_args=CommonUDFArgs(
+                resource_request=None,
+                batch_size=None,
+            ),
         )(self)
 
     def is_null(self) -> Expression: