title: self.opt_op = self.optimizer.minimize(self.loss) TypeError: minimize() missing 1 required positional argument: 'var_list'
body: <p>The code can execute in tensorflow v1,but when i execute the code in tensorflow v2,it report errors:
  TypeError: minimize() missing 1 required positional argument: 'var_list'</p>
  <pre><code>def build(self):
      self._build()
      self.outputs = self.concat_vec
      self._loss()
      self._accuracy()
      self.opt_op = self.optimizer.minimize(self.loss)  
  </code></pre>
  <p>but i don't know what variable to add</p>