Add built-in GridSampler

package/samplers/grid_search/LICENSE

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+MIT License
+
+Copyright (c) 2024 Preferred Networks, Inc.
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

package/samplers/grid_search/README.md

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+---
+author: 'Optuna team'
+title: 'Grid Search'
+description: 'Sampler using grid search.'
+tags: ['sampler', 'built-in']
+optuna_versions: ['3.6.1']
+license: 'MIT License'
+---
+
+## Class or Function Names
+- GridSampler
+
+## Example
+```python
+import optuna
+
+
+def objective(trial):
+    x = trial.suggest_float("x", -100, 100)
+    y = trial.suggest_int("y", -100, 100)
+    return x**2 + y**2
+
+
+search_space = {"x": [-50, 0, 50], "y": [-99, 0, 99]}
+mod = optunahub.load_module("samplers/grid_search")
+sampler = mod.GridSampler(search_space)
+study = optuna.create_study(sampler=sampler)
+study.optimize(objective)
+```
+
+## Others
+See the [documentation](https://optuna.readthedocs.io/en/stable/reference/samplers/generated/optuna.samplers.GridSampler.html) for more details.

package/samplers/grid_search/__init__.py

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+from optuna.samplers import GridSampler
+
+
+__all__ = ["GridSampler"]