Builder is not initialized when attaching entities in a many to many relation

[federicoparroni]

Describe the bug
If you try to call the attach method to connect two model instances that are in a many to many relation, the following error is raised:

AttributeError: class model 'MyModel' has no attribute builder

To Reproduce

1. Declare two models connected in a many to many relation:

   class Ssl(Model):
       __table__ = 'ssls'
       @belongs_to_many('ssl_id', 'app_id', 'id', 'id', table='ssl_app')
       def apps(self):
           from .app import App
           return App
    
    class App(Model):
       __table__ = 'apps'
       @belongs_to_many('app_id', 'ssl_id', 'id', 'id', table='ssl_app')
       def ssls(self):
           from .ssl import Ssl
           return Ssl

2. Attach one entity to the other one:

   ssl = Ssl.find(1)
   app = App.find(2)
   
   ssl.attach('apps', app)   # Exception!
   # or simmetrically:
   app.attach('ssls', ssl)   # Exception!

This happens because the BelongsToMany class attach method access builder without checking whether it is initialized or not:

orm/src/masoniteorm/relationships/BelongsToMany.py

Line 366 in 798de75

Pivot.on(current_model.builder.connection)

One workaround is to call get_builder() before calling attach:

ssl.get_builder()
ssl.attach('apps', app)   # now it works!

but I think the proper fix would be to change the BelongsToMany method in this way:

# Pivot.on(current_model.builder.connection) 
Pivot.on(current_model.get_builder().connection)

Expected behavior
The entities should be connected in the relation without issues.

Desktop (please complete the following information):

• OS: Ubuntu
• Version: 21.04

What database are you using?

• Type: MySQL
• Version: 14.14 Distrib 5.7.30, for Linux (x86_64)
• Masonite ORM 2.6.1