Hello Yuvaraj,

def default_report_template(self):
act_model = self.env.context.get('active_model')
active_model_id = self.env['ir.model'].search([('name','=',act_model)]).id
filtered_report = self.env['ir.actions.report'].search([('binding_model_id','=',active_model_id)], limit=1)
return filtered_report and filtered_report.id or False

report_templates = fields.Many2one('ir.actions.report', default=default_report_template)

Thanks & Regards,
Email: odoo@aktivsoftware.com
Skype: kalpeshmaheshwari