[Hotfix] Create palette

[keigohtr]

What is this PR for?

Since palette is not fixed, label will be invalid for training. Moreover, prediction process does not use the colormap of training dataset.

This PR fixes these issues.

This PR includes

• Create a palette when training.
• Create a colormap when predicting.

What type of PR is it?

Bugfix

What is the issue?

N/A

How should this be tested?

N/A

[keigohtr]

Summary

• Create the fixed color_palette
• Ignore the padding area when calculating the loss
• Ignore drop_last on training when BATCH_SIZE is bigger than dataset size.
• Add accuracy score for monitoring.

[keigohtr]

Seems no problem. Thank you!