Thank you Mr. Sudhir for answering and suggesting search_read method, But  Here I using Python code I added a product.template attachment in product.product model also same as vice versa.

_name = 'ir.attachment'
    @api.model
    def search_read(self, domain=None, fields=None, offset=0, limit=None, order=None):
        if domain and len(domain) == 2:
            res_id = False
            res_model = False
            for domain_list in domain:
                if domain_list[0] == 'res_id':
                    res_id = domain_list[2]
                elif domain_list[0] == 'res_model':
                    res_model = domain_list[2]
            if res_id and res_model and res_model == 'product.template':
                product_variant_ids = self.env['product.template'].browse(res_id).product_variant_ids.ids
                if product_variant_ids:
                    domain.insert(0, '|')
                    domain.insert(1, '&')
                    domain += ['&', ['res_id', 'in', product_variant_ids], ['res_model', '=', 'product.product']]
            elif res_id and res_model and res_model == 'product.product':
                product_rec = self.env['product.product'].browse(res_id)
                if product_rec:
                    domain.insert(0, '|')
                    domain.insert(1, '&')
                    domain += ['&', ['res_id', '=', product_rec.product_tmpl_id.id], ['res_model', '=', 'product.template']]
        return super(IrAttachment, self).search_read(fields=fields, offset=offset, limit=limit, domain=domain, order=order)

For Attachment Count in template & product Added a methods like this:

_name = "product.product"
    @api.multi
    def _compute_message_attachment_count(self):
        attachment_obj = self.env['ir.attachment']
        for record in self:
            domain = ['|', '&', ['res_id', '=', record.product_tmpl_id.id], ['res_model', '=', 'product.template'],
                       '&', ['res_id', '=', record.id], ['res_model', '=', 'product.product']]
            record.message_attachment_count = attachment_obj.search_count(domain)

_name = "product.template"
    @api.multi
    def _compute_message_attachment_count(self):
        attachment_obj = self.env['ir.attachment']
        for record in self:
            domain = ['|', '&', ['res_id', '=', record.id], ['res_model', '=', 'product.template'],
                       '&', ['res_id', '=', record.product_variant_ids.ids], ['res_model', '=', 'product.product']]
            record.message_attachment_count = attachment_obj.search_count(domain)

Hi, can I ask why do you need to attach it to product.product template? Is it to view on the website?