Synchronize `grain/python/experimental.py` with `grain/experimental.py`

[bzantium]

Problem

Currently, there are two separate files meant to export the same experimental APIs:

• grain/experimental.py
• grain/python/experimental.py

However, these files are currently not synchronized, leading to inconsistencies.

This synchronization is critical because downstream libraries (e.g., MaxText) often rely on patterns like import grain.python as grain. If the exports in grain/python/experimental.py lag behind grain/experimental.py, users accessing grain via the python subpackage miss out on new experimental features or face unexpected import errors.

Proposed Solution

Refactor grain/python/experimental.py to simply import everything from the top-level experimental module:

# grain/python/experimental.py
from grain.experimental import *