fix initial_epoch in fine tuning step

[sharkfisher]

initial_epoch for the fine tuning phase should be 1 more than history.epoch[-1], so that the history_fine.epoch would be [10, 11, ...19], a total of 10 fine tune epochs.
In the current version, without the '+1', history.epoch is [0, 1, ...9], and history_fine.epoch is [9, 10, ... 19], the epoch index overlaps at 9, fine tune actually trained for 11 epochs, not 10, and the model is actually trained for 21 epochs in total (confirmed in the x axis of the combined training history curve - 21 data points). Training for 21 epochs in total is not the problem, but history and history_fine epoch index overlapping is a bug.

The 'initial_epoch' in model.fit() could be loosely interpreted as # of epochs the model has trained for prior. If a model has never been trained, initial_epoch=0 (default). If it's been trained for 10 epochs already, initial_epoch should be 10, which in this tutorial is history.epoch[-1]+1.

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[MarkDaoust]

Thanks!

[sharkfisher]

I added the formatting check and also went with an alternative fix for that initial_epoch issue, since if 'initial_epoch' is really how many epochs the model has previously been trained for, then the alternative fix is both conceptually and code-wise cleaner. I have both changes committed to my own repo. I see that it's already shown up in the history for this pull request above. Will this new version be reviewed as part of this pull request? or do i have to open a diff pull request?
(Apologize for the newbie question. My first time with such things... :P )

[sharkfisher]

Using alternative fix for cleaner concept and code.
Added formatting check

[8bitmp3]

@MarkDaoust @markmcd PTAL