Fix for issue #72

[TaleirOfDeynai]

The issue this resolves was closed without resolution, but it was bugging me enough and wasting my GPU time too much that I felt a need to fix it.

Adds an attribute to track the current index and prevents logging an image if there has already been a logging attempt at the current index.

This prevents a circumstance where the image logger can trigger more than once in rapid succession, generally when multiple iterations occur during a single global step, such as:

• When accumulate_grad_batches > 1.
• When the validation data-loader triggers at the end of an epoch, and that global step happens to align with image logging.