class "DyLoRANetwork" lacks the "is_mergeable" method

At line 1711 of `sdxl_gen_img.py`, the script calls the `is_mergeable()` method to check whether a network is mergeable. But this causes an error when using dylora, since the `DyLoRANetwork` class has no such method.

I am not sure whether a dylora is mergeable, but it seems like a simple method to implement. Thank you for your hard work!

Start at line 1711 of sdxl_gen_img.py and inspect the DyLoRANetwork class definition to understand how is_mergeable() is used. Determine whether DyLoRA networks are mergeable, then ensure the call no longer raises an error and reports the correct behavior.

Written by the indexing model from the issue text.