Sanitize label column when initializing a Datalab instance.

[elisno]

Check for nan values in the label column.

This cannot be handled by the NullIssueManager, because it occurs in Datalab(data=df_with_nan_value_in_label_column, label_name="label_column").

For now, we need better error reporting.

[01PrathamS]

@elisno @jwmueller Including code block

if np.isnan(labels).any():
    raise ValueError("Labels must not contain null values")

under cleanlab.datalab.internal.data.Multiclass.extract_labels . does this look appropriate?