contract/contract_variable_quantity/views/contract.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>

    <record id="contract_contract_form_view" model="ir.ui.view">
        <field name="model">contract.contract</field>
        <field name="inherit_id" ref="contract.contract_contract_form_view"/>
        <field name="arch" type="xml">
            <xpath expr="//field[@name='code']" position="after">
                <field name="skip_zero_qty"/>
            </xpath>
            <xpath expr="//field[@name='contract_line_ids']/tree/field[@name='quantity']" position="before">
                <field name="qty_type"/>
                <field
                    name="qty_formula_id"
                    attrs="{'invisible': [('qty_type', '!=', 'variable')]}"
                />
            </xpath>
            <xpath expr="//field[@name='contract_line_ids']/tree/field[@name='quantity']" position="attributes">
                <attribute name="attrs">{'invisible': [('qty_type', '!=', 'fixed')]}</attribute>
            </xpath>
        </field>
    </record>

</odoo>