Fix saving text encoder weights and kohya weights in advanced dreambooth lora script

[DN6]

What does this PR do?

There seems to be an issue with

1. Saving Kohya style weights in the dreambooth advanced lora script. The save path can output a weird directory when using a relative path. e.g. setting output_dir=../checkpoints/my-dog-model will result in the following line creating a path like '../checkpoints/my-dog-model/../checkpoints/my-dog-model.safetensors' https://github.com/huggingface/diffusers/blob/610a71d7d467e4cc892c824db882071ff0d282e1/examples/advanced_diffusion_training/train_dreambooth_lora_sd15_advanced.py#L1984

I've changed this to save the kohya weight as args.output_dir/kohya_lora_weights.safetensors but I'm open to any better naming.

2. When passing in the train_text_encoder argument, I believe there is a missing else statement resulting in an ValueError always being raised.

Fixes # (issue)

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