report a bug instead of crash when vae_decoder_only=True and using img2img

[chinshou]

At now if we use new_sd_ctx with vae_decoder_only=true , the img2img api call will crash without any exception catched.

We had better checking the vae_decoder_only flag in the img2img function to report ng flag and return NULL instead of crash that is much more user friendly.

[Windsander]

this too =D

[leejet]

The current API lacks extensive documentation, requiring users to have a deeper understanding of some code details. I'll find some time later to improve the documentation for certain APIs.