[16.0]partner_invoicing_mode: Not grouping anymore invoice sby sale order partner or invoice partner.

[javierjcf]

Module

Partner Invoicing Mode

Describe the bug

When this module is instaled, if you select 2 or more orders with differents customers, it creates only a invoice. Not grouped by partner either partner_invoice_id.

To Reproduce

Affected versions: 16

Steps to reproduce the behavior:

1. Create 2 sale orders with diferent customers
2. Select the two orders and generate invoiced
3. Only one invoice is created

Expected behavior
Two diferent invoice created.

Additional context
It seems a lot of changes in this module in the last months. Very different to v 15 version.
I think the method def _get_invoice_grouping_keys(self) is the problem. It removes key partner_id and adds partner_invoice_id.
Odoo does not compare the grouping keys against the sale order, instead it does with the vals of the invoice, so no key partner_invoice_id in this dicctionary, it will allways be None, this is odoo code:
for _grouping_keys, invoices in groupby(invoice_vals_list, key=lambda x: [x.get(grouping_key) for grouping_key in invoice_grouping_keys]):

invcoice_vals_list will never contain partner_invoice_id. When odoo creates this dict, 'partner_id' key is alrready partner_invoice_id of sale_order, so i think removing partner_id key is an error.