Performance issue in the definition of sact_image_heatmap, summary_utils.py

[DLPerf]

Hello, I found a performance issue in the difinition of sact_image_heatmap, summary_utils.py, tf.stack will be created repeately during the program execution, resulting in reduced efficiency. So I think it should be created before the loop in the sact_image_heatmap.

The same issues exist in:

• tf.to_float(tf.logical_not(elements_finished))
• tf.expand_dims(residual_mask, 3)

Looking forward to your reply. Btw, I am very glad to create a PR to fix it if you are too busy.