contract/contract_variable_qty_prorated/views/abstract_contract_view.xml

<?xml version="1.0" encoding="utf-8" ?>
<!--
    Copyright 2018 ACSONE SA/NV.
    License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
-->
<odoo>

    <record id="contract_abstract_contract_line_form_view" model="ir.ui.view">
        <field
            name="name"
        >Contract Line Form View (in abstract_contract_view.xml)</field>
        <field name="model">contract.abstract.contract.line</field>
        <field
            name="inherit_id"
            ref="contract.contract_abstract_contract_line_form_view"
        />
        <field name="arch" type="xml">
            <xpath expr="//field[@name='quantity']" position="attributes">
                <attribute name="attrs" />
                <attribute name="required">True</attribute>
            </xpath>
        </field>
    </record>

</odoo>