The "partner_id" is a field declared in the current model, it works as a link between this model, and the fields related to the "res.partner" model like "name" in your example
So, in order for the related field to work, you'd need to first have a field "stock_quant", then make your related field.
But in your case, I think that the real issue is the type of the related field. What i'm seeing is you're trying to create a Char field, while the "source field" is "location_id".
Try the following and it would work:

test_field = fields.Many2one(string='test', related='stock_quant.location_id)