ATO 207 run form validation on form activation

[indam23]

Proposed changes:

• Revert change here https://github.com/RasaHQ/rasa/pull/10295/files#r807244104 that removed running the form validation action on activation of the form (before the loop is active).
• This would make Add an action that runs on form start/initialization, before active_loop is set yet #10913 obsolete since the 2.x mechanism for achieving the same thing would be returned.
• Closes required_slots function in form is not called #11199
• Resolves the issue where any extract_<slot_name> method in a FormValidationAction subclass only runs after a required_slot in the form has been set (i.e. after activation).

Status (please check what you already did):

• added some tests for the functionality
• updated the documentation
• updated the changelog (please check changelog for instructions)
• reformat files using black (please check Readme for instructions)

[ancalita]

Looks good, an additional q I have is to double-check there was no need to add this check back into _get_slot_extractions for loop which was removed in PR 10295?

if not tracker.active_loop:
  break

[indam23]

double-check there was no need to add this check back into _get_slot_extractions for loop which was removed in PR 10295?

Looked into this, but it doesn't seem to make any difference whether the check is there or not; all forms tests pass either way 🤔

[ancalita]

Whoop whoop 🎉
Left one tiny comment to make sure we have that wondering assert back into test_activate too, otherwise good to merge.

[losterloh]

🚀🚀🚀

[github-actions]

🚀 A preview of the docs have been deployed at the following URL: https://11326--rasahq-docs-rasa-v2.netlify.app/docs/rasa

[XiaofeiQian]

Can we merge and release this soon? thanks.