Masanori Kanazu

Hi, I'm working on the following. They are regression metrics defined in `sklearn.metrics._regression.py`. ``` sklearn.metrics.mean_absolute_error sklearn.metrics.mean_pinball_loss sklearn.metrics.mean_absolute_percentage_error sklearn.metrics.mean_squared_error sklearn.metrics.mean_squared_log_error sklearn.metrics.median_absolute_error sklearn.metrics.explained_variance_score sklearn.metrics.r2_score sklearn.metrics.max_error sklearn.metrics.mean_tweedie_deviance sklearn.metrics.mean_poisson_deviance sklearn.metrics.mean_gamma_deviance sklearn.metrics.d2_tweedie_score sklearn.metrics.d2_pinball_score sklearn.metrics.d2_absolute_error_score ```

I reduced the scope and start working only on `sklearn.metrics.mean_absolute_error`.

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