Fix bug with KeyError: 'params/eta'

[clearhanhui]

Why are these changes needed?

Reference to #1244

Related issue number

#1244

Checks

• I've included any doc changes needed for https://microsoft.github.io/FLAML/. See https://microsoft.github.io/FLAML/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[yxtay]

Any follow up on this please? The PR looks ready.

[thinkall]

LGTM, thanks!

[thinkall]

Would you like to add more explainations to the docstring?

[thinkall]

Suggested change

	Args:
	d (dict): The target dictionary to be updated.
	u (dict): A dictionary containing values to be merged into `d`.
	Update the dictionary `d` with the dictionary `u` recursively.
	Args:
	d (dict): The target dictionary to be updated.
	u (dict): A dictionary containing values to be merged into `d`.
	Example:
	```python
	d= {'num_boost_round': 10, 'params': {'max_depth': 12, 'eta': 0.02}}
	u= {'params': {'verbosity': 3, 'booster': 'gbtree'}}
	recursive_update(d, u)
	d # {'num_boost_round': 10, 'params': {'max_depth': 12, 'eta': 0.02, 'verbosity': 3, 'booster': 'gbtree'}}
	```

[thinkall]

Check out here for code formatting. https://microsoft.github.io/FLAML/docs/Contribute#pre-commit