[Data] - Make AggregateFnV2 generic over accumulator/result (Generic[AggType, U]) (#57281)

## Why are these changes needed?
The current Generic types in `AggregateFnV2` are not tied to the class,
so they are not picked up properly by static type checkers such as mypy.

<!-- Please give a short summary of the change and the problem this
solves. -->
By adding the Generic[] in the class definition, we get full type
checking support.


## Checks
- [x] I've signed off every commit(by using the -s flag, i.e., `git
commit -s`) in this PR.
- [x] I've run pre-commit jobs to lint the changes in this PR.
([pre-commit
setup](https://docs.ray.io/en/latest/ray-contribute/getting-involved.html#lint-and-formatting))
- [N/A] I've included any doc changes needed for
https://docs.ray.io/en/master/.
- [ ] I've added any new APIs to the API Reference. For example, if I
added a
method in Tune, I've added it in `doc/source/tune/api/` under the
           corresponding `.rst` file.
- [N/A] I've made sure the tests are passing. Note that there might be a
few flaky tests, see the recent failures at https://flakey-tests.ray.io/
- Testing Strategy
   - [ ] Unit tests
   - [ ] Release tests
   - [ ] This PR is not tested :(

---------

Signed-off-by: Arthur <atte.book@gmail.com>
Co-authored-by: Goutam <goutam@anyscale.com>

Author: ArthurBook

doc/source/conf.py

@@ -32,10 +32,9 @@
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use os.path.abspath to make it absolute, like shown here.
-assert not os.path.exists("../../python/ray/_raylet.so"), (
-    "_raylet.so should not be imported for the purpose for doc build, "
-    "please rename the file to _raylet.so.bak and try again."
-)
+assert not os.path.exists(
+    "../../python/ray/_raylet.so"
+), "_raylet.so should not be imported for the purpose for doc build, please rename the file to _raylet.so.bak and try again."
 sys.path.insert(0, os.path.abspath("../../python/"))
 
 # -- General configuration ------------------------------------------------
@@ -130,6 +129,9 @@
 nitpicky = True
 nitpick_ignore_regex = [
     ("py:obj", "ray.actor.T"),
+    ("py:obj", "ray.data.aggregate.AccumulatorType"),
+    ("py:obj", "ray.data.aggregate.SupportsRichComparisonType"),
+    ("py:obj", "ray.data.aggregate.AggOutputType"),
     ("py:class", ".*"),
     # Workaround for https://github.com/sphinx-doc/sphinx/issues/10974
     ("py:obj", "ray\\.data\\.datasource\\.datasink\\.WriteReturnType"),