add Block component creation callback

[w-e-w]

Description

allow insert components using on_before/after_component_elem_id or before/after_component from extensions to before / after a Block
this is not previously possible because the callback is not called for Blocks
this PR allows this

Example

insert befor and after txt2img_hires_fix_row4

stable-diffusion-webui/modules/ui.py

Line 333 in f0f100e

with FormRow(elem_id="txt2img_hires_fix_row4", variant="compact", visible=opts.hires_fix_show_prompts) as hr_prompts_container:

insert hear

like so

Demo extention script

from modules import scripts
import gradio as gr


class Script(scripts.Script):
    before_cb = None
    after_cb = None
    
    def title(self):
        return "block insert component demo"

    def show(self, is_img2img):
        if not is_img2img:
            self.on_before_component_elem_id = [
                ('txt2img_hires_fix_row4', self.before_hires_fix_extended_ui),
            ]
            self.on_after_component_elem_id = [
                ('txt2img_hires_fix_row4', self.after_hires_fix_extended_ui),
            ]
            return scripts.AlwaysVisible

    def ui(self, is_img2img):
        if self.before_cb and self.after_cb:
            return [self.before_cb, self.after_cb]

    def before_process(self, p, *args):
        print(*args)

    def before_hires_fix_extended_ui(self, *args, **kwargs):
        with gr.Accordion(label='Before'):
            self.before_cb = gr.Checkbox(label='Before Checkbox')

    def after_hires_fix_extended_ui(self, *args, **kwargs):
        with gr.Accordion(label='After'):
            self.after_cb = gr.Checkbox(label='After Checkbox')

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• My code follows the style guidelines
• My code passes tests