bug if NOT self.config.grad_clip

[zyxue]

FYI, https://github.com/zlpure/CS234/blob/master/assignment2/q2_linear.py#L245

would produce error as clipped_grads_and_vars wouldn't be available if not self.clipped_grads_and_vars

[zlpure]

Thanks! if NOT self.config.grad_clip, clipped_grads_and_vars is not defined. But the performance will be better when using grad clipping. So I omit the self.config.grad_clip flag.