Remove metrics placeholder from training loop

[tfujiwar]

What this patch does to fix the issue.

In train.py, metrics_summary_op is an operator which takes metrics_values as a placeholder. metrics_values comes from sess.run(list(metrics_ops_dict.values())).

metrics_values = sess.run(list(metrics_ops_dict.values()))
metrics_feed_dict = {placeholder: value for placeholder, value in zip(metrics_placeholders, metrics_values)}	

metrics_summary, = sess.run(	
    [metrics_summary_op], feed_dict=metrics_feed_dict,	
)

This logic exists because we calculated metrics outside networks before. But now metrics calculation is a part of graphs. Essentially, we can execute metrics_summary_op directly without any placeholders.

Link to any relevant issues or pull requests.

A part of #481 .

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[iizukak]

@tfujiwar nice refactoring.
Ownership Approval

[tfujiwar]

/ready

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