Enable PNGinfo for textual inversion training.

[timntorres]

Fulfills #3186. The requested changes are entirely symmetric to #3189, #3309, and #3625; just for embeddings instead of hypernets. I ran some light testing and can confirm these changes work under the following cases:

✅with "Save text information about generation parameters as chunks to png files" both enabled and disabled ✅with "Create a text file next to every image with generation parameters" both enabled and disabled