Add empirical attainment surface

[nabenabe0928]

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[y0z]

Suggested change

	"package/visualization/plot_empirical_attainment_surface"
	"visualization/plot_empirical_attainment_surface"

[y0z]

I got the following error.

Traceback (most recent call last):
    ax = plot_multiple_empirical_attainment_surfaces(
        multiple_study_list, levels=[3, 5, 7], labels=["Random", "TPE"]
    )
TypeError: plot_multiple_empirical_attainment_surfaces() missing 1 required positional argument: 'attainment_ratios'

[nabenabe0928]

Thanks, I renamed the argument, but forgot updating here!

[nabenabe0928]

@y0z
Could you review this PR again?

[y0z]

@nabenabe0928 Thank you for your update.

Then, I encountered another issue.

The documentation says len(attainment_ratios) can be 1.

 `attainment_ratios`: The ratios of studies that need to dominate the surface. The length must be either 1 or 3. If the length is 1, the corresponding attainment surface will be visualized.

However, I got the following error with attainment_ratios=[0.5].

    ax = plot_multiple_empirical_attainment_surfaces(
        multiple_study_list, attainment_ratios=[0.5], labels=["Random", "TPE"]
    )
  File "/Users/yozaki/.cache/optunahub/github.com/optuna/optunahub-registry/f222e6887cc0b2651d9caa827f73a26437bcaf80/package/visualization/plot_empirical_attainment_surface/_wrapper.py", line 100, in plot_multiple_empirical_attainment_surfaces
    ax.legend()
    ^^^^^^^^^
AttributeError: 'list' object has no attribute 'legend'

[nabenabe0928]

@y0z
Thank you for the catch, I fixed the issue!

[y0z]

LGTM

[y0z]

@nabenabe0928
Although this is optional, providing a thumbnail/generated figure example may be helpful.